ABC : ZYX :: CBA : ?

XYZ
BCA
YZX
ZXY

The answer is $\boxed{\text{B}}$.

In the sequence $ABC:ZYX$, the first letter of the first term is followed by the third letter of the second term, and the second letter of the first term is followed by the second letter of the second term. This pattern is repeated to generate the third term, $ZYX$.

Applying this pattern to the sequence $CBA$, we get $BCA$.

Option A, $XYZ$, is incorrect because the first letter of the first term is followed by the second letter of the second term, and the second letter of the first term is followed by the third letter of the second term. This is the opposite of the pattern in the original sequence.

Option B, $BCA$, is correct because it follows the pattern in the original sequence.

Option C, $YZX$, is incorrect because the first letter of the first term is followed by the third letter of the second term, and the second letter of the first term is followed by the first letter of the second term. This is not the pattern in the original sequence.

Option D, $ZXY$, is incorrect because the first letter of the first term is followed by the second letter of the second term, and the second letter of the first term is followed by the third letter of the second term. This is the opposite of the pattern in the original sequence.