Can I say,

(a) She is mopping the kitchen floor.

(b) He is mopping the floor of / in the kitchen.

(c) He is mopping in the kitchen.

(d) He is mopping the kitchen.
All OK!
Thanks. But my friend told me , after the verb "mopping",it is followed by the object.

So, we have to say:

She is mopping the kitchen. (But not "mopping in the kitchen")

What do you think?
Teachers: We supply a list of EFL job vacancies
I think you should not listen to your friend's English grammar advice.
But, my friend is Rachel, moderator. I think you also know her well.
That is what she tole me.

Don't use "in" with "mopping."
"Mopping" just takes an object, in this case "kitchen."
It's like "cleaning the kitchen."

Rachel, Moderator,
Ah. Well, Rachel is good, but I would certainly argue with her on this one. Except that she does not permit arguing at her site.

You can indeed use 'mopping in/under/around/etc'-- the object (almost always 'floor') is understood:

My wife was mopping around my feet, so I kicked her.
She was mopping under the bed and found my other slipper.
I was mopping in the kitchen, so I didn't hear the phone ring.

If you can point me to the thread, I'll have a word with Rachel. Or maybe-- just maybe-- you misunderstood her.
Students: We have free audio pronunciation exercises.
Thanks. Can I say,

He is mopping floor / a floor.
Unusual but possible:

He is mopping a floor.
Vincent TeoThanks. Can I say,

He is mopping floor / a floor.

He is mopping the floor.
Students: Are you brave enough to let our tutors analyse your pronunciation?