In this sentence, the word "highly" is an adverb modifying the verb "thinks", describing the intensity or degree of Mr. Brown's thoughts about himself. The adverb form "highly" is needed to show how Mr. Brown thinks, rather than the adjective "high" which describes a noun.
B. highly
In this sentence, the word "highly" is an adverb modifying the verb "thinks", describing the intensity or degree of Mr. Brown's thoughts about himself. The adverb form "highly" is needed to show how Mr. Brown thinks, rather than the adjective "high" which describes a noun.