Here we know that $\angle M N O$ is a parallelogram, with lines $\(\angle O\)$ and $M N$ being parallel and $\(\angle M\)$ and $O N$ being parallel.
Therefore, all the angles opposite one another in the parallelogram are equal (angle $\(O L M\)$ is equal to $\(M N O\)$ ). With this information, we know that angle $\(\angle M N\)$ is equal to $\(\angle O N\)$.
Even though we don't know the exact value of these angles, we know that

$$
\(\begin{aligned}
& a-17=b+4 \\
& a=b+4+17
\end{aligned}\)
$$

$\(a=b+21\)$, so angle $a$ is greater than $b$ and $(A)$ is the correct answer choice.