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

Correct — A. The trigger is the clause after the adjective: that competitors quickly emulated it.

A that-clause stating the result of a degree needs so + adjective + that. The approach was effective to such a degree that copying followed.

'Their approach proved so effective that competitors quickly emulated it' → option (a).

Why the others are wrong

• (b)
  too — Too means more than is wanted and pairs with a to-infinitive: too marginal to influence policy. It does not introduce a that-clause of result.
• (c)
  as — As needs a second as to complete it: as effective as its rivals. 'As effective that' pairs it with the wrong word.
• (d)
  very — Very intensifies and nothing more. 'Very effective' is fine alone, but very sets up no result, so the that-clause is left hanging.

English marks a degree and its consequence with so ... that: so + adjective or adverb, then a that-clause saying what followed.

With a noun the frame is such ... that: such an effective approach that rivals copied it.

Too also marks degree, but it means excess and takes a to-infinitive: too marginal to influence policy. As ... as compares two things as equal. Very links to no clause.

Key facts

• so + adjective + that-clause: so effective that competitors emulated it.
• such + noun phrase + that-clause: such an effective approach that competitors emulated it.
• too + adjective + to-infinitive: too marginal to influence policy.
• as + adjective + as: as effective as the old method.

Common traps

• Choosing too because it also marks degree. Too pairs with to + verb (too marginal to influence), not with a that-clause.
• Choosing very because 'very effective' sounds right on its own, without reading the that-clause to the end.

No directly related past PYQ was found.