To answer this question, the user needs to have knowledge of the JCL (Job Control Language) used in mainframe systems.

The correct answer is option C, PROC.

Explanation:

In JCL, PROC is used to define a procedure. A procedure is a set of JCL statements that can be reused in multiple jobs.

When defining a procedure, default values can be assigned to the parameters defined in the procedure. These default values are used if the parameter values are not specified in the job step that calls the procedure.

Therefore, option C, PROC, marks the beginning of an in-stream procedure and assigns default values to parameters defined in the procedure.

Option A, JOB, is used to identify the start of a job and specify job-level parameters.

Option B, EXEC, is used to identify the start of a job step and specify step-level parameters.

Option D, PEND, is not a valid JCL keyword.

Option E, None of the above, is incorrect as the correct answer is option C, PROC.

Thus, the answer is: C. PROC