Select the option that correctly replaces the underlined portion of the given sentence to make it grammatically sound:

The committee members retreated back to their initial proposal following the debate.

The correct option is retreated to their initial proposal.

Explanation:
The original sentence contains a grammatical error known as redundancy (or tautology) in the phrase "retreated back".

1. Eliminating Redundancy:
The verb "retreat" inherently means to move back, go back, or withdraw from a position. Adding the adverb "back" right after "retreated" creates unnecessary repetition. Removing "back" corrects the redundancy.

2. Retaining the Possessive Pronoun:
The possessive pronoun "their" is required to show ownership and specify whose proposal is being discussed (the committee members' proposal). Removing "their" leaves the sentence awkward and grammatically incomplete.

3. Analysis of the Options:
• retreated to initial proposal: Incorrect because it drops the necessary possessive determiner "their".
• withdrew back to their proposal: Incorrect because "withdrew back" is also redundant ("withdraw" means to step back).
• retreated to their initial proposal: Correct because it removes the redundant word "back" while keeping the proper possessive pronoun "their".
• moved back to initial proposal: Incorrect because it omits the determiner "their".

Therefore, the grammatically sound sentence is: "The committee members retreated to their initial proposal following the debate."