EXHAUSTING or draining. Let us take a look at what the options MEAN:Weaken: make or become weaker in power, resolve, or physical strength.Attack: take aggressive military action against.Surmount: overcome (a difficulty or obstacle).Destroy: end the EXISTENCE of (something) by damaging or attacking it. From the above words, it is clear the 'weaken' is closest in meaning to debilitate as it refers to a state of being or making weak.An EXAMPLE of the same is: It is EXPECTED that the illness will debilitate him for at least another week. Therefore 'weaken' is nearest to the word 'debilitate.'