I believe that the problem has been transcribed incorrectly and should read as follows:

Scientists now suspect that imperial mammoths of the Pleistoscene period were wiped out not by human predation, as were mammoths of earlier periods, but by a sudden and extreme change in climate.

a) as were mammoths of earlier periods
b) like that in an earlier period
c) as in earlier periods
d) as were earlier periods
e) like the mammoths in an earlier period

The original sentence conveys the following comparison:
Imperial mammoths of the Pleistocene period were wiped out not by human predation, as were mammoths of earlier periods [wiped out by human predation].
The bracketed words are omitted but implied.
The conveyed meaning is that two sets of mammoths -- imperials mammoths of the Pleistoccene period and mammoths of earlier periods -- were wiped out in opposite ways:
Whereas imperial mammoths of the Pleistocene period were NOT wiped out by human predation, mammoths of earlier periods WERE wiped out by human predation.

In B, that lacks a clear singular antecedent.
Eliminate B.

Generally:
as serves to compare CLAUSES.
If the clause after as lacks a subject and verb, then the subject and verb from the first clause are implied in the clause after as.

C: Imperial mammoths of the Pleistocene period were wiped out not by human predation, as in earlier periods
Here, the clause after as lacks a subject and verb.
As a result, the subject and verb from the first clause are implied in the clause after as.
The resulting comparison is nonsensical:
Imperial mammoths of the Pleistocene period were wiped out not by human predation, as imperials mammoths of the Pleistoscene period were wiped out by human predation in earlier periods.
Eliminate C.

D: Imperial mammoths of the Pleistocene period were wiped out not by human predation, as were earlier periods
Implied comparison:
Imperial mammoths of the Pleistocene period were wiped out not by human predation, as were earlier periods wiped out by human predation.
The portion in red is nonsensical.
Eliminate D.

Generally, COMMA + like serves to refer to the preceding SUBJECT.
E: Imperial mammoths of the Pleistocene period were not wiped out, like the mammoths in an earlier period
Here, the portion in red seems to refer to imperial mammoths -- the preceding subject -- implying that imperial mammoths of the Pleistocene period were LIKE the mammoths in an earlier period.
The intended meaning of the original sentence is that the two sets of mammoths were DIFFERENT, as noted above:
Whereas imperial mammoths of the Pleistocene period were NOT wiped out by human predation, mammoths of earlier periods WERE wiped out by human predation.
Eliminate E.

The correct answer is A.