Straight "D" - as this is the only option which is talking the difference between within the "Related class" - Rest all options are talking about different classes or unrelated classes...
i don't think this is 700 level question

D it is; definitely not 700. An analogy all children like ice cream; i like ice cream so i must be a child ? no
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Hi domfrancondumas, I hope this helps.

A. makes an unwarranted assumption that a characteristic that is shared by two classes of things is their only common feature. For this answer choice, it states, "shared by two classes of things." In this argument, there is only one class of thing mentioned which is the orchid flower.

B. assumes without warrant that a characteristic distinguishes one class of things from another is the only characteristic characteristic that distinguishes that class from the other. This answer choice is incorrect because in this argument, we are concerned with the similarity not differences.

C. mistakes a characteristic that is unique to one particular class of things for a characteristic that is unique to an unrelated class of things. This answer choice is incorrect because the argument DOES NOT link one class of things with other class of things.

D. Treats a characteristic known to be true of one class of things as if that characteristic were unique to that class. This answer choice is correct because the argument assumes that any orchid species that are pollinated solely by insects have features that attract insects.. Since the recently discovered orchid has this characteristic, the argument concludes that the recently discovered orchid is pollinated solely by insects.

E. makes broad generalizations about all members of a particular class of things on the basis of what is known about a member of an unrelated class of things. This answer choice is incorrect because the argument uses generalization of particular class to apply to specific thing in that class. So, this is actually reversed.
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This question follows a typical patter that assumes since all things of a certain type share a characteristic that all things with that characteristic are of the certain type.

To take it out of the subject at hand consider the argument. All children wear shoes. The person I am talking to is wearing shoes, so she must be a child.

in this context the argument is clearly lacking because we understand that lots of people, not just children, wear shoes. The argument is doing the same with orchids but that subject is less familiar to us.

it is saying all orchids that are insect pollinated have features that attract insects. This orchid has a feature that attracts insects, therefore it must be insect pollinated. I hope the above analogy helps you to understand this type of reasoning.