As a result of the continuing decline in the birth rate, less people

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

Since people are "countable", fewer is the correct usage. A, B eliminated.
Use of "as" is incorrect because we are comparing. C, D also eliminated.
E is correct, "than" is required.

we do not need to discuss about comparison to get OA for this problem. but, we can discuss comparison for other questions.

some special comparion sentence require zero subject in the second clause of comparison. look at choice e.
we can not add subjects for "than did...." . specifically, we can not add "number of persons," or, "persons" or "they", because all of these subjects will make wrong comparison. so this special pattern requires zero subject.
I give you another question from gmatprep.
According to a 1996 survey by the National Association of College and University Business Officers, more than three times as many independent institutions of higher education charge tuition and fees of under $8,000 a year than those that charge over $16,000.

(A) than those that charge
(B) than are charging
(C) than to charge
(D) as charge
(E) as those charging

this special pattern is the way for gmat to trick us. but no longer can it do so if we are informed

The word 'less' can only be used for countable nouns. This leaves us with only choices C, D, and E.
The usage of 'as' for comparison between two things is grammatically incorrect.
This leaves us with choice E, and that is the correct answer.

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