difficult, hard, laborious, tough
difficult, strenuous,
Both are correct.
Synonym means: a word having the same or nearly the same meaning as another in the language. Definition means: the act of defining or making definite, distinct, or clear / the formal statement of the...
He is a pronoun. The only synonym it has is to use the name of the person it refers to, i.e., "He is short." "John is short."