Option $b$.
The eigenvalues are found by solving the characteristic equation. For a matrix $M_n$, the degree of the characteristic polynomial/equation will be $n$ and hence, there will be exactly $n$ roots. These roots can be real or complex. But we know that for an equation with real coefficients, complex roots (if any) will exist in conjugate pairs. So, for an odd value of $n$, even if there exist complex roots, there will be at least one real root.