This question is actually from an LSAT practice test (Prep Test 29, Section 4), and though not all the LSAT CR are applicable to GMAT CR, this Assumption one makes a good challenge! Set a timer for 2 minutes, try it on your own, then scroll down for the explanation!

Conservationist: The population of a certain wildflower is so small that the species isheaded for extinction. However, this wildflower can cross-pollinate with a closelyrelated domesticated daisy, producing viable seeds. Such cross-pollination could result in a significant population of wildflower-daisy hybrids. The daisy should therefore be introduced into the wildflower’s range, since although the hybrid would differ markedly from the wildflower, hybridization is the only means of preventing total loss of the wildflower in its range.

Which one of the following is an assumption on which the conservationist’s reasoning depends?

(A) The wildflower currently reproduces only by forming seeds.
(B) The domesticated daisy was bred from wild plants that once grew in the wildflower’s range.
(C) Increasing the population of the wildflower will also expand its range.
(D) Wildflower-daisy hybrids will be able to reproduce.
(E) The domesticated daisy will cross-pollinate with any daisy like plant.

 
Conclusion: Daisy should be introduced to wildflower’s range. Hybrid is ONLY means to prevent “total loss” of wildflower.

Evidence: Wildflower headed for extinction. Can produce seeds with daisy & “could” make hybrids.

Assumptions: Wildflower-daisy hybrids won’t be extinct as well? Wildflower can’t cross-pollinate with others? Cross-pollination will work to save wildflower?

Question Rephrase: What needs to be true for the conclusion to be true?

Prediction: Something to add credence to the “only means” and “total loss.”

(A) If the wildflower reproduces only by forming seeds, then it strengthens the author’s claim that hybridization is the “only means.”

(B) If the daisy descends from the wildflower, then it’s range would not be a “total loss.”

(C) The relationship between “population” and “range” is not the focus of the argument.

(D) If they can reproduce, then the wildflower would live on.

(E) We need to focus on the wildflower.

This is a challenging question, since both (A) and (D) seem to strengthen the argument. So what “needs” to be true? When (D) is negated (“hybrids NOT able to reproduce”), then the conclusion (“hybrids result in significant population”) will NOT hold. Thus, (D) is vital, and the correct answer.