Choose the correct preposition to fill in the blank in the sentence given below.
The scientist spent decades in the laboratory in search ________ a breakthrough cure.

The correct option is of.

Explanation:
In English grammar, certain fixed phrases (idiomatic collocations) require specific prepositions to function correctly. The phrase "in search of" is a standard fixed prepositional phrase used to express trying to find or discover something.

Breakdown of the Sentence:
• The noun "search", when preceded by the preposition "in", is always followed by the preposition "of" to specify the object or goal being sought (e.g., in search of truth, in search of a cure).
• Therefore, "in search of a breakthrough cure" correctly completes the thought.

Analysis of Other Options:
• towards: Refers to physical or abstract direction, which is incorrect in this context.
• for: While we can use "for" with the verb form (e.g., "searching for a cure"), the noun phrase "in search" strictly requires "of", not "for".
• at: Typically indicates a specific position, place, or time, making it grammatically incorrect here.

Thus, the complete sentence reads:
"The scientist spent decades in the laboratory in search of a breakthrough cure."