Final answer:
The common monthly extension increment for re-enlistment in the military is 6 months, (option B) but it may vary depending on the specific branch and circumstances.
Step-by-step explanation:
When eligible for re-enlistment, service members can typically extend their enlistment contract in various increments as per the needs of their military service. Among the options available, 6 months (B) is a common increment for monthly extensions. However, this may vary between different branches of the military and can depend on the specific circumstances or requirements of service at the time of re-enlistment. Service members should check with their unit's personnel officer or re-enlistment NCO to get the most up-to-date information regarding their options for extension.