This question checks the literal Sanskrit meaning of the word yoga, which is often confused with just physical exercise.
Union: The word yoga comes from the Sanskrit root yuj, which means to join, unite or yoke together. It refers to the union of the individual self with the universal consciousness, or the joining of body, mind and spirit.
Separation: This is the opposite of what yoga means. Yoga is about bringing things together, not pulling them apart, so this option is wrong.
Exercise: In everyday modern usage, yoga is often treated as a form of physical exercise involving postures and breathing. However, that is a common practical use of yoga, not the literal meaning of the Sanskrit word itself.
None of the above: Since Union is a correct and well known meaning, this option is ruled out.
The correct option is Union, the literal Sanskrit meaning of yoga.
Let's summarize:
The Sanskrit root yuj means to join or unite.
Yoga therefore literally means union, not simply physical exercise.