Watching the TV or working on the computer leads to overeating.

Ans:A
(A) Watching the TV or working on the computer leads to overeating.correct
(B) Watching the TV or working on the computer lead to overeating."lead" wrong. for or we use the subject after or for subj-verb agreement..
(C) To watch the TV and to work on the computer leads to overeating.and always plural."leads" is wrong
(D) Watching the TV and working on the computer leads to overeating.same as c.
(E) Watching the TV or to work on the computer leads to overeating. not parallel

C has more trouble than a compound subject. We don't want to say "To X leads to Y." How can we use "To X"?

*"To X is to Y." We use this to equate two activities: "To live is to suffer." "To make that joke is to tell half of your audience that you do not respect them."

*To X is Y. We use this to evaluate one activity. "To live is a great joy." "To make that joke was very foolish." (Go easy on these--many such cases can be better expressed in other ways.)

*To X, Y. In this case, the infinitive at the front is now a modifier. (In the previous cases it was the subject.) We are saying that if you want to achieve X, you must do Y: "To earn a high score, you must master time management." Note that in everyday (correct) English, we often employ this construction with imperatives that do not include a subject: "To access the stairs, turn right." This is fine, but you're not likely to see this on the GMAT.

So we don't want to say "To watch TV" at all. If we do combine it with another infinitive, as C does, then it's true that we should have a plural verb. But note that if we had a conjunction *within* the infinitive, that wouldn't require a plural: "To live in Manhattan and never leave home is a waste of money." (That's why so many of my NYC students have spent the past 2 years scattered back to their childhood homes! )

As a side note, *all* the answers commit an error in saying "watch the TV." (Or to use an infinitive, "To say 'to watch the TV' is an error.") When we describe a general activity, it should be "watch TV." Otherwise, it sounds like we are specifying a specific TV. You'd never ask someone "How often do you watch the TV?"