Final answer:
The mathematical operation that must be performed to obtain the resultant when two vectors of different magnitudes pointing in the same direction are added is Addition.
Step-by-step explanation:
The mathematical operation that must be performed to obtain the resultant when two vectors of different magnitudes pointing in the same direction are added is Addition (option D). When adding vectors with the same direction, we simply add their magnitudes together to find the resultant vector. For example, if vector A has a magnitude of 3 and vector B has a magnitude of 5, the resultant vector would have a magnitude of 3 + 5 = 8.