Final answer:
In a synchronous communication with a BPEL process, a partner web service typically requires both an input message and an output message to facilitate a two-way interaction.
Step-by-step explanation:
If a partner web service of a BPEL process communicates with the BPEL process synchronously, the service will typically require to have both an input message and an output message. This is because synchronous communication implies a two-way conversation, where one service sends a request (input message) and waits for a response (output message) before proceeding. This interaction pattern is common in web services communication, where operations usually involve request-response behaviors.