This question requires selecting the correct words to complete the sentence: "He was one of my best ________ and I felt his loss ________". We need to consider the grammar rules for plural nouns and adverbs.

Grammar Analysis for Blank 1

The phrase "one of my best..." indicates that the subject ('He') is part of a larger group. Therefore, the noun following 'best' must be plural.

• Option 1 and 3 use the singular noun 'friend'. This is incorrect because the structure implies multiple friends.
• Option 2 and 4 use the plural noun 'friends'. This correctly fits the grammatical structure.

Grammar Analysis for Blank 2

The second blank needs a word to describe *how* the loss was felt. This requires an adverb to modify the verb 'felt'.

• Option 1 and 4 use the adverb 'keenly', meaning intensely or sharply. This correctly modifies 'felt'.
• Option 2 and 3 use the adjective 'keen' or the comparative adjective 'keener'. These do not fit grammatically to modify the verb.

Identifying the Correct Option

Combining the analysis for both blanks:

• The first blank requires the plural noun friends.
• The second blank requires the adverb keenly.

Therefore, the correct combination is friends, keenly.

The completed sentence reads: "He was one of my best friends and I felt his loss keenly."