A large pharmaceutical company has recently developed

To weaken the argument that continued production of the antibiotic will lead to the extinction of the tutu fish, we need to find information that suggests an alternative source for the scales or counters the claim that the continued production will lead to extinction.

Let's evaluate each of the answer choices:

(A) The antibiotic made from the tutu fish's scales is only available by prescription.

This answer choice does not provide any information about the tutu fish population or an alternative source of the scales. It's unrelated to the argument.
(B) Because tutu fish are rare, they are also expensive.

This answer choice supports the argument by reinforcing the idea that tutu fish are rare, which suggests that using their scales for the antibiotic could lead to their extinction. It does not weaken the argument.
(C) The inhabitants of Sando Island regard the meat of the tutu fish as a delicacy.

This answer choice does not directly address the argument regarding the extinction of tutu fish due to antibiotic production. It is unrelated to the argument.
(D) Tutu fish breed well in captivity, and can be cultivated on fish farms.

This answer choice weakens the argument significantly. If tutu fish can be bred in captivity and cultivated on fish farms, it means that there is an alternative source for obtaining the scales needed for the antibiotic production. This suggests that the continued production of the drug may not necessarily lead to the extinction of the tutu fish.
(E) Sando Island is difficult to reach since it does not have an airstrip.

This answer choice is unrelated to the argument and does not provide any information about the tutu fish population or the production of the antibiotic.
The answer choice that most seriously weakens the argument is (D) because it suggests an alternative source for obtaining the scales of tutu fish, which undermines the claim that the continued production of the drug will lead to their extinction.