Correct Answer: B

Explanation:
Enable millisecond (msec) timestamps using the service timestamps command:

router(config)#service timestamps debug datetime msec.
router(config)#service timestamps log datetime msec The “service timestamps debug”.

command configures the system to apply a time stamp to debugging messages. The time-stamp format for datetime is MMM DD HH:MM:SS, where MMM is the month, DD is the date, HH is the hour (in 24-hour notation), MM is the minute, and SS is the second. With the additional keyword msec, the system includes milliseconds in the time stamp, in the formatHH:DD:MM:SS.mmm, where .mmm is milliseconds.