Each sentence below is split into four parts. Choose the part containing the grammatical error.

It was highly rude of the committee chair to reject the volunteer's proposal without discussion.

The correct option containing the grammatical error is: It was highly rude of the.

Explanation:
The grammatical error lies in the usage of the modifier "highly" with the adjective "rude".
In English grammar, the adverb "highly" is an adverb of degree that has specific collocational preferences. It is typically paired with adjectives denoting degree, probability, or status (e.g., highly likely, highly recommended, highly qualified, highly effective, highly respected). It is not conventionally or idiomatically used to modify simple behavioral or emotive adjectives like rude, bad, or polite.
To intensify the adjective "rude", appropriate adverbs of degree such as "very", "extremely", "incredibly", or "exceedingly" should be used instead.

Corrected Sentence:
"It was very rude of the committee chair to reject the volunteer's proposal without discussion."