The tails of dolphins and crocodiles are different in structure but similar in function. what are these kinds of structures called?

The correct answer would be analogous organs.

Analogous organs refer to the set of organs which are different in origin but perform the same function due to convergent evolution.  

For example, wings of the birds and wings of insects are different in origin but perform the same function, that is, flying.

Similarly, tails of dolphins and crocodiles are different in structures but similar in function, thus, they form a pair of analogous organs.

In contrast, homologous organs refer to the set of organs which are similar in origin but perform different function due to divergent evolution.  

For example, arms of humans and wings of birds are made up of a similar set of bones but perform different functions.

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