Aclass of word that describes another word is called a : a) verb b) modifier c) part of speech d) conjunction

A verb is an action word.
He walked to school. Walked is the verb in the sentence. It implies doing something.

A Conjunction is a word the connects clauses together. They can be major or minor conjunctions. 
He walked to school and he met two of his friends sitting on the school steps.
And connects 2 main clauses. It is a major conjunction. It joins equal things.

Because it was raining, he was soaked. because joins a subordinate clause (because it was raining) with the major clause, it is a minor conjunction.

It is a part of speech but there is a better more specific answer

B<<<<< All modifiers describe another word.

To get what you want you have to work for it.