While global temperatures have risen sharply over the last century, they have only recently eclipsed the Medieval period, during which scientists theorize that a series of volcanic eruptions sent the earth into a period of historic warmth.
A. they have only recently eclipsed the Medieval period, during which scientists theorize that a
B. they have only recently eclipsed those of the Medieval period, during which scientists theorize that a
C. it has only recently eclipsed the Medieval period, during which, scientists theorize, a
D. it has only recently eclipsed that of the Medieval period, during which scientists theorize a
E. they have only recently eclipsed those of the Medieval period, during which, scientists theorize, a
Subject-Verb Agreement: While looking at the answer choices, we have a 2v3 split (it has vs they have). Considering the subject of the sentence (global temperatures) is plural, the corresponding pronoun must also be plural i.e.
they have. This helps in eliminating options C and D.
Logical Comparison: In option A, there is an incorrect comparison between temperatures and the Medieval period. The intended meaning of the sentence is to compare the temperatures in two different periods.
This leaves us with options B and E.
Tense/meaning: Both options use the verb 'theorise' in its present tense. However, while interpreting the meaning of the sentence in Option B, the Medieval period (which was obviously in the past) does not correspond to the present tense 'theorise'.
Option E, by using the right punctuation, gives the intended meaning of the sentence - that scientists currently theorise about the events during the Medieval Period. Ans E.