Explanation
Correct answer: B.
Choice B is the best answer because as used in the text, a span of time is described as "a void" space, which most nearly means an empty or vacant one. In the text, the speaker describes summertime in counterintuitive terms: although nature’s "surging life is at its full" during the season, the speaker feels summer to be "a time of pause, / A void and silent space between two worlds." The speaker says further that during summer, "feeling sleeps / Gathering strength" for future efforts. Thus, the speaker regards summer as an empty stretch of time, to be followed by a period of greater activity.
Why the other choices are wrong
Choice A
Choice A is incorrect. Although the text does present summer as a time of inactivity, it doesn’t characterize that inactivity as useless, or as having no purpose; in fact, the speaker regards summer as a time when "feeling" gathers "strength for efforts yet to come."
Choice C
Choice C is incorrect. Although the text characterizes summer as a time "when inspiration lags, and feeling sleeps," it doesn’t discuss the season’s relationship to the speaker’s memory or suggest that summer can easily be forgotten.
Choice D
Choice D is incorrect. In some contexts, "void" can mean devoid of, or lacking, a particular element, and such a lack could be conceived of as incompleteness. However, the text doesn’t portray summer as not being complete or whole; instead, it characterizes vacancy or inactivity as being an essential quality of the season, as experienced by the speaker.