Select the correct option: Their approach proved ______ effective that competitors quickly emulated it.

Explanation:

• The structure “so … that” is used to show a result or consequence.
• The sentence expresses that the approach was very effective, with the result that competitors copied it.

Sentence:
Their approach proved so effective that competitors quickly emulated it.

Why other options are incorrect:

• Too → used with “to” (too … to), not with “that”
• As → used in comparison (as … as), not for result
• Very → intensifies but does not form the correct structure with “that”

Extra facts:

• “So + adjective/adverb + that + result” is a common English structure.
• Example: “She was so tired that she fell asleep.”

✔ Final Answer: So