Concept:The structure “so + adverb + that” is used to express a result or consequence. The adverb must remain in its base form.
Explanation:The sentence shows that the boy ran at such a speed that the defenders could not catch him.
Only the “so … that” pattern correctly links the action to its result.
Option A, “very fast,” is incorrect because “very” cannot be followed by “that” to express a consequence.
Option B, “much faster,” is a comparative phrase, so it does not fit the “so … that” structure.
Option C, “so faster,” is grammatically wrong because “faster” is a comparative adverb and cannot replace the base adverb “fast.”
The base adverb “fast” must be used after “so.”
Therefore, the correct phrase is “so fast that.”
The full sentence becomes: He ran so fast that the defenders couldn’t catch up with him.
Answer:D. so fast