Hi abhasjha,

This SC is built around a Comparison (which also requires Parallelism rules). There is also an Idiom/Usage issue, which we'll deal with first.

1) Idiom (Countable vs. Noncountable) - The answer choices provide us with 2 options: "smaller" (which is a word that defines the physical size of something) or "less" (which is a word that defines a comparison that is NOT physical). Since the prompt refers to "commitment...", that is a noun that is NOT physical - the word "less" is the proper idiomatic word. Eliminate A, C and E ("lessening" isn't appropriate in either case).

2) Comparisons - In a Comparison, you MUST compare "like" things.

We can compare people to other people
We can compare what people have to what other people have
We can compare a personality trait to another personality trait
Etc.

The first part of the sentence describes what young people HAVE....so the second part of the sentence must describe what older people HAD. Eliminate B.

Final Answer: D

GMAT assassins aren't born, they're made,
Rich