Select the correct option:
The committee meets on Monday, ___ the final decision will be announced.

The correct option is when.

Let us analyze the grammatical context and meaning of the given sentence:

The sentence is: "The committee meets on Monday, ___ the final decision will be announced."

1. Understanding Relative Adverbs & Conjunctions:
The blank introduces a relative clause modifying the time specified in the first clause ("on Monday"). The word needed must act as a relative adverb referring to time.

2. Analyzing the Options:
• when: Functions as a relative adverb referring to a point in time ("on Monday"). It correctly connects the two clauses to indicate that the final decision will be announced at that time (on Monday).
• whereupon: Means "immediately after which", which is overly formal or archaic and less natural in this context.
• at then: Grammatically incorrect in English.
• on which: While preposition + relative pronoun combinations can indicate time (e.g., "on which day"), "when" is the most direct, standard, and appropriate relative word used to join these two clauses cleanly in this sentence context.

Therefore, the complete correct sentence is: "The committee meets on Monday, when the final decision will be announced."