The correct answer is option 'B': more.

Explanation:
In this sentence, we are comparing the intensity of the pain the person experienced with the level of pain he could bear. When we compare two things, we use the comparative form of an adjective.

The comparative form of an adjective is used when we want to compare two things and indicate that one thing has a higher degree or level of a quality than the other. In this case, we are comparing the pain the person experienced to the level of pain he could bear.

Examples of comparative adjectives:
- The pain was more intense than he could bear.
- She is taller than her sister.
- This book is more interesting than the one I read last week.

In the given sentence, the adjective "much" (option 'A') is an adverb, not an adjective. Adverbs are used to modify verbs, adjectives, or other adverbs, but they cannot be used to compare two things. Therefore, "much" is not the appropriate form of the adjective to complete the sentence.

The adjective "most" (option 'C') is the superlative form of an adjective. Superlative adjectives are used when we want to compare more than two things and indicate that one thing has the highest degree or level of a quality. In this sentence, we are comparing only two things (the pain experienced and the pain that could be borne), so the superlative form is not appropriate.

Therefore, the correct form of the adjective to complete the sentence is "more," making the sentence: "The pain was more than he could bear."

The Answer is b