B is the only right answer. "Foolproof" is indeed the correct word, meaning it is proofed against dumb things that might happen due to foolishness. Some people think the word is "fullproof." It's proofed against full? Sorry, I don't get it.

The others:

• A features the botched version of the phrase "cut off the nose to spite your face." It's a well-known saying but some people think the "to spite" is "despite." Nope!
• C features "tow the line," but you can't tow the line--hey, where are you taking it? You toe it, as in, approach it, stand where you're supposed to.
• D features the word "easedrop," which doesn't exist. Some people think "eavesdrop" is "easedrop," though I can't imagine why. Basically it just refers to how people used to stand "under the eaves" to listen--the "eavesdrop" is just the name of the area. It became a verb.