A = uu*
where u is a 3x1 matrix
so, u* is a 1x3 matrix
The key to this question is AB and BA share non-zero eigenvalues
Lets say, B = u*u
So, B will be a 1x1 matrix
Hence, B will have only 1 eigenvalue
Therefore, Out of 3 eigenvalues of A, only 1 (sharable by B) will be non-zero. Other 2 eigenvalues will be 0.
Answer is 1