What typically determines which retirement pay plan applies to a service member?

The correct answer is based on the significance of the date of initial entry into military service, which plays a crucial role in determining the retirement pay plan that applies to a service member. This date essentially marks the beginning of a service member's career and is fundamental in distinguishing which retirement system they fall under.

For example, service members who entered military service before a certain date may be eligible for the Legacy Retirement System, while those who joined thereafter could be subject to the Blended Retirement System or other modern plans. The specific retirement benefits and calculation formulas can vary significantly based on this initial entry date, influencing the overall retirement package a service member will receive.

The other options, while relevant in different contexts, do not directly dictate the retirement pay plan. Factors like the date of birth, number of years served, or retirement plan election may influence pay amounts or choices within the system but do not inherently establish which retirement system a member belongs to. Thus, the date of initial entry into military service is the determining factor regarding the applicability of retirement pay plans for service members.