Choose the correct word: He is ___ weak.

The correct word to intensify the adjective 'weak' in this context is "very". 'Very' is used to emphasize the degree of an adjective or adverb without implying excessiveness or a negative connotation.

• A: too implies an excessive or undesirable degree (e.g., 'too weak to stand').
• B: much is typically used with uncountable nouns (e.g., 'much water') or in comparative structures (e.g., 'much weaker'). It's not used directly before a positive adjective like 'weak'.
• C: very is the appropriate intensifier for adjectives like 'weak', indicating a high degree.
• D: so can intensify, but often implies a consequence (e.g., 'so weak that he fell') or is used in specific emphatic structures. 'Very' is the most straightforward intensifier here.

Choosing the correct intensifier depends on the nuance and grammatical context.