As a result of the continuing decline in the birth rate,
less people will enter the labor force in the 1980’s than did in the 1960’s and 1970’s, a twenty year period during which people born after the war swelled the ranks of workers.
Main comparison among
people who will enter the labor force in
1980's,
1960's and 1970's.
Underlined portions starts with
"less" people - we know that - people - countable noun, hence need
"fewer". Eliminate (A) and (B).
Also we need correct constraction for comparison
"fewer...than", so eliminate (C) and (D).
If we delve little bit deeper,
"will be entering" in (B) and (D) - future continious tense - we need some point of time in the future to refer, so is incorrect.
(A)
less people will enter the labor force in the 1980’s than
(B)
less people will be entering the labor force in the 1980’s
as(C) fewer people will enter the labor force in the 1980’s
as(D) fewer people
will be entering the labor force in the 1980’s
as(E) fewer people will enter the labor force in the 1980’s than
E is the answer