The purpose of requiring source code escrow in a contractual agreement is to:

 According to the How Important Is Source Code Escrow - ISACA article, the purpose of requiring source code escrow in a contractual agreement is to ensure that the source code isavailable if the vendor ceases to exist. Source code escrow is the deposit of the source code of software with a third-party escrow agent, who releases it to the licensee only if certain conditions are met, such as the bankruptcy, merger, or acquisition of the licensor. This arrangement protects the licensee from losing access to the software support and maintenance, and allows them to continue using and modifying the software as needed. Therefore, the answer is B. ensure that the source code is available if the vendor ceases to exist. References = How Important Is Source Code Escrow - ISACA