I was debating between D and E. I will go with E.

with D

D here.

Anandnk, remember that the question is not asking for criticism of Kant's doctrine but "of the speaker's use of Kant's ethical doctrine".

OA is E. This is an LSAT question and I also missed this one. The official explanation is as follows:

The speaker relies on a key element of Kant's ethical theory to reach a conclusion about the ethical treatment of animals. But is the theory applicable to animals? Choice E answers that question by saying that the theory, taken on its own terms, applies only to people. Choice A is perhaps the third-best answer, for it can be read to suggest that Kant's failure to consider the possibility somehow vitiates the use made of the theory by the speaker. But the fact that the original proponent did not see all possible implications of a theory doesn't mean that the theory itself does not have those implications. Choice B is a fairly weak answer, unless you try to read it as saying those who use animals for food and clothing are behaving unethically because they give offense to others, but that seems a fairly attenuated reading. Choice C is pretty obviously irrelevant to the question of whether the argument can be applied to animals. And choice D may be the second-best answer, because it suggests, at least, that animals don't think -- they just eat. But to the extent that you want to use choice D to make that kind of argument(and there's nothing wrong with making a creative argument in school), in the testing environment, you should quickly see that choice E is the better, more direct route to the same conclusion
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Best Regards,

Paul