Solution

From statement a:

All $Z$s are $Y$s.

From statement b:

No $Y$ is a $X$.

So, if all $Z$s are $Y$s and no $Y$ is a $X$, then no $Z$ can be a $X$.

Hence, conclusion II follows.

Now, statement c says:

Every $X$ is a $W$.

But there is no direct relation given between $Z$ and $W$.

So, conclusion I does not follow.

Therefore, only conclusion II follows.