SAT Question aa443c4b: Form Structure And Sense answer and explanation

Choice A is the best answer. The convention being tested is the use of verb forms within a sentence. The nonfinite present participle "suggesting" is correctly used to form a restrictive participial phrase ("suggesting...Holocene") within the main clause ("This hypothesis...cannot stand..."). This participial phrase functions as part of the sentence’s subject ("This...Holocene"), providing essential identifying information about what the hypothesis states—namely, that certain trees survived without interruption or human intervention throughout the Holocene.