This question tests the correct usage of plural nouns and adverbs in a sentence.

Best Friends Grammar

The phrase "one of my best ________" requires a plural noun.

• "friend" is singular.
• "friends" is plural.

Since the phrase refers to a single person from a group of multiple friends, the plural form friends is grammatically correct. Thus, the first blank should be filled with friends.

Felt Loss Adverb Usage

The phrase "I felt his loss ________" requires an adverb to modify the verb "felt".

• "keen" is an adjective (e.g., a keen mind).
• "keener" is the comparative form of the adjective (e.g., keener insight).
• "keenly" is the adverb form, meaning deeply or intensely.

To describe how the loss was felt, an adverb is needed. Therefore, keenly is the correct word for the second blank.

Completed Sentence

Combining the correct words, the sentence becomes: "He was one of my best friends and I felt his loss keenly."