The answers are:
1. c some eggs. Egg is a countable noun, whereas the other three are uncountable.
2. b. gulp. To gulp is to eat or drink food or liquid quickly and in large amounts, whereas the other three means only to drink an amount of liquid.
3. b. You don't have to wear a suit. This sentence means 'you are not required to', whereas the other three mean 'you are prohibited to'.
4. b. He was drunk by 6 o'clock. 'By' here followed by an expression of time, whereas the other three 'by' followed by a doer or an agent of the action.
Anticipating Learner's difficulties:
1. All these five phrases show different degrees of certainty, from the least certain (may well go) to the most certain/definite (will definitely go). Students, particularly if English is their second language, often find this difficult to understand.
2. All these three sentences are idioms. Students may translate the meaning of an idiom word for word, which of course will not make any sense, since an idiom is a group of words in a fixed order that have a particular meaning that is different from the meanings of each word understood on its own.
3. certain adjectives can only go with certain prepositions. This is called collocations. For example, students may say 'interested with' instead of 'interested in', or 'keen about' instead of 'keen on'.
I hope these explanations answer your questions.
Good luck on your CELTA course!