You cannot memorise enough words, and you do not have to. A few dozen Latin and Greek roots, prefixes and suffixes cover a large share of the words these questions use, and they let you reason about a word you have never seen.
The second technique is to put the word in a sentence you would actually say. A word that fits your sentence and a word that does not is a faster test than trying to recall a definition.
Try each one before opening the answer. A solution you have read is not a solution you can produce under time pressure, which is the only kind that counts.
Q1 Choose the synonym of BENEVOLENT: (a) hostile (b) kind (c) wealthy (d) cautious.
(b) kind.
"Bene" means good or well - the same root as benefit, benign and benefactor - and "volent" is wishing. Benevolent is well-wishing, so kind. "Hostile" is its antonym, which is why it is offered first.
Q2 Choose the antonym of TRANSPARENT: (a) clear (b) obvious (c) opaque (d) fragile.
(c) opaque.
"Trans" is through and "parent" here is appearing - transparent is what light passes through. Opaque is precisely the opposite. "Clear" and "obvious" are synonyms offered to catch somebody answering the wrong question.
Q3 Choose the synonym of GARRULOUS: (a) silent (b) talkative (c) angry (d) generous.
(b) talkative.
If the word is unfamiliar, the root helps: it shares an origin with "garrulity" and relates to chattering. It describes someone who talks too much. "Silent" is the antonym, again placed first.