Which type of modification is signed by both the contractor and the contracting officer?

A bilateral modification is the correct answer because it requires the agreement and signature of both the contractor and the contracting officer. This type of modification is typically used when both parties agree to change the terms of the contract, whether it's adjusting the price, scope of work, or schedule. The mutual consent inherent in a bilateral modification ensures that both parties are on the same page and have formally accepted the changes.

In contrast, a unilateral modification is executed by only one party, usually the contracting officer, and does not require the contractor's consent, although the contractor may be required to comply with the changes. Default modifications generally relate to situations where a contract is terminated due to the contractor's failure to perform and are not necessarily signed by both parties. Convenience modifications allow for changes made by the contracting officer for convenience and typically do not involve contractor agreement in the same way as bilateral modifications do.