Fill in the blanks with the most suitable word from the options given…

The correct answer is Option (3) → because

Conjunction of reason:

• Because introduces the reason for what is said in the main clause.
• This sentence: the desire for revenge is dangerous, and the reason is that it does not diminish with time, so "because" joins the two.
• Complete sentence: The desire for revenge is very dangerous because it does not diminish with time.
  Example: We stayed indoors because it was very hot.

Why the Other Options are Incorrect?

Option 1: whereas — Incorrect. Whereas shows contrast between two facts; the second clause explains the first, it does not contrast with it.
Example: Riya likes tea, whereas her sister prefers coffee.

Option 2: while — Incorrect. While shows time (during) or contrast; neither fits a clause that gives a reason.
Example: She listened to music while she worked.

Option 4: so — Incorrect. So introduces a result; "dangerous, so it does not diminish" turns the reason into a result, which reverses the logic.
Example: It was raining, so we stayed indoors.

Contrast: "because" gives a reason, "so" gives a result, and "whereas" shows a contrast.