Difficulty Level: Hard

Explanation is given below

Some species of dolphins find their prey by echolocation;
they emit clicking sounds and listen for echoes returning from distant objects in the water.

Speculation: Those same clicking sounds might have a second function: particularly loud clicks might be used by the dolphins to stun their prey at close range through sensory overload.

We need to cast doubt on the correctness of the speculation. So we are looking for something that says that the clicking sounds may not serve this second purpose; they may not stun the prey.

(A) Dolphins that use echolocation to locate distant prey also emit frequent clicks at intermediate distances as they close in on their prey.
Irrelevant. Talks about how the clicks are used for prey location. Doesn't say anything about whether they are used for stunning too.

(B) The usefulness of echolocation as a means of locating prey depends on the clicking sounds being of a type that the prey is incapable of perceiving, regardless of volume.

We know that dolphins use echolocation to locate prey. This is given to us. This option tells us that echolocation can be used to locate prey only if the prey is incapable of perceiving the sound (Makes sense. If the prey perceives the sound, it will swim away from that location). Since the dolphins do use echolocation to local prey, it stands to reason then that the prey can't perceive the sound. If the prey cannot perceive the sound, how will they get a sensory overload? They will not be able to sense the sound at all. Hence, this option casts doubt on the speculation.

(C) If dolphins stun their prey, the effect is bound to be so temporary that stunning from far away even if possible, would be ineffective.

Irrelevant. The argument talks about stunning at close range. What happens from far away is irrelevant.

(D) Echolocation appears to give dolphins information about the richness of a source of food as well as about its direction.

Irrelevant. Doesn't address the speculation at all.

(E) The more distant a dolphin's prey, the louder the echolocation clicks must be if they are to reveal the prey's presence to the hunting dolphin.

Irrelevant. Doesn't address the speculation at all. It again deals with how echo sounds are used to locate prey. It doesn't say imply about stunning.

Answer (B)