Answer:
b. time dilation
Step-by-step explanation:
Time dilation is the change in time which is observed when an object is moving near the speed of light. Time flows slower for people which are moving. Time dilation is given by the equation.

where,
= Moving observer's period of the clock
= Time passed in frame of the clock itself.
v = Speed at which the person is moving
is always greater than
.
So, here when one twin returns the other twin is 60 years old is due to time dilation.