Step 1: Read the logic of the passage.
The passage talks about how each generation looks down on the next, forgetting how it was to be young. Adults laugh at their own past indiscretions, but criticize the current youth for the same things — showing hypocrisy.
Step 2: Fit the blanks.
- First blank: “successive generation” makes sense as generations come one after another.
- Second blank: They forget what it feels like to be “young.”
- Third blank: It is the “adults” who think back on their youth.
- Fourth blank: Their reaction is “hypocritically,” as they condemn the same actions they once did.
Step 3: Eliminate wrong choices.
- (A) “cautiously” doesn’t fit the tone.
- (B) Repetition of “young” is awkward.
- (D) “naturally” doesn’t capture the hypocrisy.
- (E) “sincerely” is opposite to the idea.
Thus, Option C fits perfectly.
Final Answer:
\[
\boxed{\text{C}}
\]