Which of the following is the best example of an informal word that means difficult

-arduous

-onerous

-struggle

-tough

tough

Explanation:

"Tough" may be formal or informal depending upon the situation in which it is used.

Example:

He was a tough customer to deal. (informal).

He had a really tough time. (formal)

However "onerous" and arduous are very formal words meaning "difficult".

Meaning of "struggle" is not "difficult". It means effort, endeavor, labor. Struggle is a noun, while "difficult", "arduous", and "onerous" are all adjectives (modifying or describing a noun or pronoun).