Which of the following, if true,
most helps to explain the surprising finding?
(A) Tidal currents do not themselves dislodge barnacles from the shells of horseshoe crabs.
Ok, but does it answer our question "helps to explain surprise finding"? No, Eliminate
(B) Barnacles most readily attach themselves to horseshoe crabs in areas where tidal currents are weakest.
In question it says horseshoe crab has negligible barnacles population even in weak tidal current. Doesn't help us to find surprise finding, though it is helping to surprise even more now.
(C) The strength of the tidal currents in a given location varies widely over the course of a day.
we have given weak currents in 2nd findings, it mat vary but still remain weak comparatively to tidal current in 1st finding
(D) A very large barnacle population can significantly decrease the ability of a horseshoe crab to find food.
"very large" to be noted here. then it could have moderately large in second finding but that is not the case. Eliminate
(E) Until they are fully grown, horseshoe crabs shed their shells and grow new ones several times a year.
Yes that's correct. if they shed shells every now and then how they will collect barnacles.