From the question stem it looks like an inference question

(A) Some part of the panthers’ current habitat is only of marginal quality.
Irrelevant and not mentioned anywhere in the passage.

(B) If the population of Florida panthers ever exceeds 250, it will be self-sustaining.
It is directly mentioned in the passage.

(C) Unless Florida panthers acquire a larger habitat, their population will not be self-sustaining.
Can be inferred from the passage. Till the time population doesn't reach 250 its not self sustaining and in current habitat the population cant grow hence in current habitat population cannot be self-sustaining. Correct.

(D) The population of Florida panthers will never increase much beyond its current level.
if the size of the habitat increases then the population can increase. Incorrect.

(E) Today, Florida panthers occupy a larger habitat than they did in the 1970s.
Cant be concluded from the passage. Maybe the habitat was bigger and Panthers were dying due to another reason like hunting.

Hence C is the correct answer.
_________________

"The population must reach at least 250" is one condition which should be true in order to make the population self-sufficient.

In an option B

there is an assertion: if this condition is true, the population will be self-sufficient......Hence, B is incorrect because there could be other factors may affect to make the population self-sufficient.

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