A matrix is diagonal if
In other words, you want non-zero elements only when the indices of the row and the column are the same: the first element of the first row must be non zero, the second element of the second row must be non-zero, and so on. All other elements must be zero.
Answer:
Vertical opposite angles are the same so the answer is D.
Plug into the equation of the ellipsoid:
Complete the square:
Then the intersection is such that
which resembles the equation of a circle, and suggests a parameterization is polar-like coordinates. Let
(Attached is a plot of the two surfaces and the intersection; red for the positive root , blue for the negative)
