On the Job - English Professions

Only Answer 4 has all the right details in place ('an ... / a ... / or a ...').

You need to have these the right way round; and remember that since they each start with a vowel, it's easier (and right!) to say 'AN ...' before the title of the job.
Remember also that English almost always says someone is 'a pilot' or 'an organist' (or whatever). Many other languages don't do the 'a'. Russian simply says 'Svetlana ballerina', without even having a verb; German says such things as 'Mein Bruder ist Lehrer', and in French 'Sa femme est actrice'. Not a hint of 'a' anywhere!

Any of these might be true, but Answer 4 offers the widest selection.

These are all good institutions, but Answer 2 is much the most likely.

A 'pharmacist' sounds as if he (or she) might be connected with farming, but that isn't true: he or she will be working in a chemist's shop, carefully preparing people's pills and medicines.
A motor mechanic may well spend time repairing tractors, but probably not so much 'using' the tractor for its main purpose, which would be working on the land and fields.
'Tractor' (in the original Latin) means 'something that pulls or drags', but we do not use that in English to mean the power-unit at the front of a train. We usually call that an 'engine' or 'locomotive'.

This is the usual expression for such a shop in English.

We need the 'a' again here, and Answer 3 is the best. If someone is a waiter or waitress, they will spend some of their time in the kitchen (at the 'pass', where the food is ready to be taken into the dining room) ... and some of it actually taking the orders, serving the food and clearing the tables when people have finished. (This is what we call 'front-of-house' in public places like restaurants and theatres.) At any rate, the waiting staff would not spend 'most of their time in the kitchen', which is what the question said.

In both the jobs that were mentioned, people have to do these two things (among much else)!

Answer 3 is also fine, though most people nowadays say 'college' rather than 'university' (or perhaps, the young people themselves shorten it to 'Uni'.)
'Veterinary surgeon' is the full version of what 'vet' stands for, but it's a great big 'mouthful' of words to say, and most people use the short form here ('I'm just taking the dog round to the vet').

A baker is someone that bakes bread: almost every country in the world must have such people.
It is worth noticing that quite a lot of English-language family names come from the jobs that people did in an earlier generation ('the butcher, the baker, the candlestick-maker' in one children's rhyme; 'tinker, tailor, soldier, sailor' in another). If you know of famous British people with names like this, it may help you remember them. (Margaret Thatcher was Britain's first woman Prime Minister towards the end of the 20th century; her husband's family name 'Thatcher' means 'someone who makes roofs for houses, using straw'. You may have seen ~ at least in pictures, or in film or on television ~ little country cottages, and other buildings, with roofs like this, which almost look more like hair!)
Other family names may come from where people live (like Street, Church, Lake, Rivers, Hill or Bridge), or some characteristic of the people (Armstrong, Whitehead, Smart), or from a name in an earlier generation (McDonald, O'Reilly, Prichard, Robertson). Maybe there are similar patterns in the names in your language, or in the phone-book at home. Once you know these things, you can make more connections in your mind, and your learning should be faster and richer.