Find the error in the underlined sentence and answer the correct one from the…

Concept: In a sentence-correction question, a modifying clause has to do three things at once — attach unambiguously to the exact noun it describes (not to some other clause nearby), agree with that noun in number if the clause has a finite verb, and use the right article: a definite article ('the') when the modifier points back to something specific already established, or none/an indefinite article when it makes a general claim. A relative clause introduced by 'that' ties directly back to its antecedent; a bare '-ing' participial phrase can leave the antecedent unclear.

Applying it here: The noun being modified is 'conditions' — the state created by the peat holding water and keeping the strand humid. The modifying phrase must show unmistakably that it is these conditions (not the broader 'which' clause before it) that produce tree growth, must agree with 'conditions' in number, and must point to the specific trees growing in this environment rather than trees in general. The relative clause "that enable the trees to grow" satisfies all three requirements together: 'that' ties the clause directly to 'conditions', the plural verb 'enable' agrees with the plural 'conditions', and 'the trees' specifies the trees just described instead of trees generally.

Checking the alternatives by value:

• "enabling trees to grow." — drops the article before 'trees', so the clause reads as a general statement about tree growth rather than one tied to this specific setting.

• "for the trees to grow." — replaces the relative clause with a purpose construction; 'for...to' states an aim rather than directly asserting what the conditions do, and reads as unidiomatic in this position.

• "for growing trees." — ambiguous between 'trees that are already growing' and 'so that trees can grow', and carries the same unidiomatic 'for' as the previous option.

Result: The relative clause "that enable the trees to grow" is the only phrasing that is simultaneously unambiguous, correctly agreeing in number, and correctly specific — it is the best correction.