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I cannot remember the actual "rules" on these following sentences and hope that some grammarians here can offer some help -

Which of the following is/are correct and for those that are not correct, can someone help with the actual grammar rule explaining why they are not correct? Thank you.

(a) What do you think is the best way to talk to her?
(b) What do you suggest is the best way to talk to her?
(c) What do you suggest to be the best way to talk to her?

My recollection of the rule pertinent to these sentences is this
(a) = What is the best way to talk to her? = What, do you think, is the best way to talk to her?

However, by this "rule", shouldn't (b) be correct also?
(b) What, do you suggest, is the best way to talk to her?

Somehow my feeling tells me (c) is correct, but (b) isn't.

Any help? Thank you very much.
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SeraphinHowever, by this "rule", shouldn't (b) be correct also?
(b) What, do you suggest, is the best way to talk to her?
Yes, it should be correct. And it is. You don't need the commas, of course.

(c), however, is awkward (to my ear).

CJ
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Here is a new blog post which dives into the details of that question: https://www.englishforums.com/blog/the-following-is-or-are/

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Hi,
Adding to CJ's comment, this may be helpful to you. Which - in the context that you used, is always singular, unless specified. It suggested "the one" among a group of items or things. For example:
Which house is yours? (you friend came to visit you and the last time he came was 3 years ago. He was circling a few times and finally called you)
Which color is your favorite? ( you are chatting with a few friends on colors..)
SeraphinWhat is the best way to talk to her? = What, do you think, is the best way to talk to her?
If you are asking for directions in the New York subway, i.e. Can you tell me which /what train goes to Brooklyn ?
What - is unspecified, it could be one, or more than one.
What are your favorite colors / hobbies ? Plural
What is your name/ sign / hobby? Singular, However, it is very common to say "What are the choices? in a restaurant.
For my "train" example, since we assumed there is only one train that goes to Brooklyn, we customarily use singular verb "is".
In short, whether a sentence requires a singular or plural verb is depending on the context and semantics.
 doriscornago's reply was promoted to an answer.
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doriscornago

Here is a new blog post which dives into the details of that question: https://www.englishforums.com/blog/the-following-is-or-are/

Nobody on this thread asked a question about 'is' or 'are' with the word 'following'. In fact, the presence of 'following' is irrelevant to the principle governing the "is/are" designation. 'which' is the subject which must agree with the verb, not 'following'.

The form "is/are", as well as analogous forms, is correctly used informally when the writer doesn't know whether one or more of a set of possibilities has the given property and wishes to signal this incomprehension in some way.

Which of the following (questions) is/are correct? implies that the writer doesn't know how many of them are correct. It may be one; it may be more than one. Both 'is' and 'are' are possible because 'which' may refer to one or to more than one referent.

Which of these programs has/have high priority?
Which of the activities mentioned above is/are considered dangerous?
Which of the world's stock markets benefits/benefit from China's growth?

Formally, you would see the singular in such cases.

CJ