The correct answer is $\boxed{\text{A. Only conclusion I follows}}$.
The first statement is “All birds are tall.” This means that every bird is tall. The second statement is “Some tall are hens.” This means that there are some tall things that are also hens.
The first conclusion is “Some birds are hens.” This conclusion follows from the first statement, because if all birds are tall, and some tall things are hens, then some birds must be hens.
The second conclusion is “Some hens are tall.” This conclusion does not follow from the first statement and the second statement. It is possible for there to be some tall things that are hens, without there being any hens that are tall. For example, all giraffes are tall, and some giraffes are birds. However, not all birds are tall, and not all tall things are birds.
Therefore, only conclusion I follows.