The correct answer is $\boxed{\text{B. zero}}$.
A singular matrix is a square matrix that has at least one zero eigenvalue. This means that there exists at least one nonzero vector $v$ such that $Av=0$. In other words, the matrix $A$ does not have an inverse.
The eigenvalues of a matrix are the roots of its characteristic polynomial. The characteristic polynomial of a singular matrix is a polynomial of degree at least one with at least one root equal to zero. Therefore, at least one eigenvalue of a singular matrix must be zero.
The other options are incorrect because they are not always true for singular matrices. A singular matrix can have positive, negative, or imaginary eigenvalues, but it must have at least one zero eigenvalue.