God, the questions people come up with! Off the top of my head, I can't think what they are trying to teach you. Maybe they are splitting hairs that don't exist in normal conversation. The truth is: all the suggestions are correct. The most usual response for 1. would probably be "Not yet" 3. No (sorry, Sergey). You can't. You could say, "I think (that) the variant a is right". Why "the"? Because it is in this particular case. The answer to the/this question is (that) the variant a is correct (another question could have the variant b as the correct response). As an addition, people would really say, "I think a is right." "The variant" would be added by an English teacher, George Eliot or possibly a mathematician. ;)

Your answer 'b' to 1 is correct. It is because there is nothing about going in the question. Use been to describe completed visits. If you have visited a place on holiday and then returned you have been there. If someone visits a place but has not come home they have gone there. so for 2 both 'a' is the correct choice by you. In 3, you cannot use 'an' with 'variant' because an is followed by a word starting with a vowel or an alphabet sounding like a vowel. In my humble view the correct form should be "n my view the variant 'a' is right.

Hello Sergey, 1. Strictly speaking, neither of the options are the true answer to the question. The question asks if the other person has already consumed or eaten lunch, in which case the correct answer would either be a. "No, I haven't yet." or b. "No, I haven't had lunch yet." or c. "No, I haven't eaten yet." If the question was "Have you been to lunch yet?", then either of your two options will be the correct answer, but you are correct in that the use of "been" sounds more natural. 2. Both are perfectly acceptable in everyday speech, but yes, the use of "been" is again more natural. 3. You'll notice I used the word "option" quite a bit when referring to the choices for the answers. It's more normal to say "In my view, option a (or b) sounds right." Apologies for the lengthy explanation.