Isn't it? Because you are talking about the fact. It's a fact,...,isn't it? If you said it another way like, 'People are trying hard to be happy, aren't they?' Without 'it is a fact that' in the beginning, that would be correct.

Choice 1 is best because the writer is asking whether or not the statement is a "fact", not whether or not "all people actually want to be happy." The second choice does not answer the implied question - "Is it a fact....?" If choice 2 were correct, it would read: All people are trying to be happy, correct/right?

I think both sound correct depending on the message you wish to convey. It is a fact that all people are trying hard to be happy,...isn't it? - this sounds correct if you are asking if someone agrees if this is a fact. The fact is the context of the sentence and is stressed. It is a fact that all people are trying hard to be happy,...aren't they? - this sounds correct if the people are the main context of the sentence. That's my thoughts.

aren't they is correct for your context. isn't it = is only correct if the adjective "happy" was not present and the question was not about "they" = people. It is a fact that all people are trying hard to be happy, aren't they? = correct It is a fact that all people are trying hard to be happy, isn't it? = incorrect. the tag question relates to the end of this sentence (people TRYING HARD to be HAPPY aren't they?) not the fact mentioned at the beginning of the sentence. It is a fact that E=MC2 isn't it? = correct we are asking a question about the fact that E does = MC2 the answer is neither automatically isn't it? nor is it automatically aren't they, a certain amount of logical thinking is required.

the correct tag is: Isn´t it?