Answer is (D) only.

This is a weaken question. There is a plan - to help the economy, kill tiger sharks within a mile of the beaches.

We have to weaken it i.e. we have to prove that killing the tiger sharks will not help the economy.

The argument tells us the following:
Tiger sharks feed on smaller sharks but sometimes attack tourists.
Tenare's fishing industry provides most revenues and tourism industry is second.

A. Even if not all the tiger sharks that come close to the beaches are killed, the existence of the program would reassure tourists.
This doesn't weaken our plan, it actually strengthens it. Tourists will get reassured and will come back. Economy will be helped.

B. Business owners who depend on tourism are willing to pay most of the cost of implementing the program.
Again, if anything, it strengthens our plan (very slightly though!). The cost of the program will not be paid by the govt and hence the revenues generated will not be lost on implementing the program. So economy will be helped.

C. Tourists come to Tenare Island for its beaches, even though the island features a number of other tourist attractions.
Irrelevant

D.The small sharks on which tiger sharks prey feed on fish that are commercially important to the island\'s fisheries.
If tiger sharks are killed, small sharks will prosper which means the number of fish they feed on will decrease. Since this fish is commercially important, if their availability reduces, the revenues from fishing may reduce. Hence, actually, the economy may not be helped. This weakens our plan.

E. Not all tourists who come to Tenare Island enjoy swimming or surfing.
Irrelevant.

Answer (D)