Identify the parts of speech of the underlined word in the following sentence:
He is intelligentenoughto solve this sum.

In the given question 'enough' is used as 'an adverb'.
The adverb 'enough' is used to modify an adjective.
In this case it goes after the adjective it modifies.
For example:
- She is old enough to earn a living.
- He was kind enough to lend me a few dollars.
According to the above rule and example 'intelligent' in the given question is an adjective that will be modified by an adverb 'enough'.
Thus, option 4 will be the correct answer.