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Hi. Please help.

1. I think the article before the underlined phrase in the no. 1 sentence is optional, but you will see more sentences without the article than the ones with it.

This group consists of (the) people of different nationalities who have immigrated to this country.

2. Could we put the definite article before the quoted content?

IMF is an abbreviation for (the) "International Monetary Fund."

3. Let's say the following question is to be answered while looking at a picture. Should it be a definite or indefinite article before the word "palace"?

Q: Where are they?
A: They are in the (a?) palace.
Comments  
1. No article. Putting "the" in makes it ungrammatical.

2. This is correct with or without the "the."

3. An article is required here, but which one would depend on the context. For example:

A: This is a video of our parents' trip to Buckingham Palace. Here they are standing in front of it. Here they are at the entrance.
B: Where are they now? I don't see them.
A: They are in the palace. ["The" is used because the palace has been identified and is well-known.]

Teacher: Class, what is this picture of?
Student: The people are in a doctor's office.
Teacher: What is this picture of?
Student: They are now in the living room of a house.
Teacher: What is this picture of?
Student: They are in a palace. ["A" is used because the palace has not been identified. It could be any palace.]
Thank you very much. As for the example sentence below, would you say putting the definite article before the wordss "great men" makes the sentence to mean all the great men of the day?

He befriended the great men of his day.

If it does, does the definite article in front of the word "people" in one of the original sentences in the first post mean all the people of different nationalities who have immigrated to that country? Thank you in advance for your help.

This group consists of the people of different nationalities who have immigrated to this country.