When must the contracting officer amend the solicitation?

The correct answer highlights the necessity for the contracting officer to amend the solicitation when changes to requirements occur. This is vital to ensure that all potential bidders are provided with updated information that accurately reflects any modifications to the project's scope, specifications, or any pertinent circumstances that could impact the proposals.

Amending the solicitation when there are changes in requirements ensures fairness and transparency in the procurement process, allowing all bidders to base their proposals on the same, current information. This process is in line with maintaining the integrity of the competition and upholding the standards of the contracting process, allowing for an efficient and equitable selection of proposals.

In contrast, simply limiting amendments to before or after the receipt of proposals would not account for the dynamic nature of requirements in many contracting situations. Moreover, while bidders requesting modifications can inform the contracting officer of potential changes, it is the responsibility of the officer to assess those requests within the context of overall project requirements rather than solely acting on them. Thus, it is the occurrence of changes in requirements that serves as the fundamental trigger for the need to amend the solicitation.