Which one of the following words is a synonym for the word hard in sentence 1?

a. Easy

b. Difficult

c. Hollow

d. Solid

Since you didn't give us the sentence 1, I can only assume.

A synonym is a word that has the same meaning as another word. So, the word hard can be synonymous with difficult, if it is a difficult task, or it can be solid, if the sentence is about rocks, or something like that.