Choose the correct word to fill in the blank: His argument was _______…

Answer: so

Rule used: Use so + adjective/adverb + that + result when the first part shows degree and the second part shows the result.

• Here, persuasive is the adjective.

• The clause that it changed everyone's perspective gives the result.

• So the correct construction is: His argument was so persuasive that it changed everyone's perspective.

Why the other words do not fit:

• too usually pairs with to + verb, not with this “that” clause.

• very only intensifies the adjective; it does not link to the result clause.

• much is not used naturally before the adjective “persuasive” in this sentence.

Therefore, the correct word is so.