Identify the correct word from the given options, which means the following :
One who collects postage stamps.
a) Philatelist
b) Stamp seller
c) Vendor
d) Lexicographer

Hint: This question is a one-word substitution question. It is the use of one word in place of a wordy phrase in order to make the sentence structure clearer.

Complete answer:
In the given question, we are asked to give a word for the phrase. The phrase is “one who collects postage stamps”. Stamp collecting is generally accepted as one of the areas that make up the wider subject of philately, which is the study of stamps. A philatelist may, but does to have to collect stamps.
Now, let us examine all the given options and find out the correct answer:
Option ‘a’ is Philatelist. He is the one who collects stamps and this option is the correct option.
Option ‘b’ is Stamp seller. A stamp seller is someone who sells stamps, he may not necessarily collect them, hence, it cannot be the correct option.
Option ‘c’ is Vendor. A vendor is someone who offers something for sale(which may or may not include stamps); especially a trader in a street.
Option ‘d’ is Lexicographer. A person who compiles dictionaries is known as a lexicographer. It has no connection with stamps, so we'll eliminate this option straight away.
As we have examined all the given options, we can easily figure out the correct answer. Here, the correct word for “one who collects postage stamps” is “philatelist”.
So, the correct answer is “Option a”.

Note:
Don’t confuse yourself between a “stamp seller” and a “philatelist”. A stamp seller may have postage stamps but he may not collect them, he may have a job of keeping stamps but this won’t make him a collector.