Which of the following is not a type of matrix?

The correct option is Minor matrix.

To understand why "Minor matrix" is not a type of matrix, let us define each of the terms mentioned in the options:

1. Diagonal Matrix:
A diagonal matrix is a square matrix in which all elements outside the main diagonal are zero. For a matrix A = [a_ij], it is a diagonal matrix if a_ij = 0 for all i ≠ j.

2. Scalar Matrix:
A scalar matrix is a diagonal matrix in which all the elements along the main diagonal are equal to the same scalar value. For a matrix A = [a_ij], it is a scalar matrix if a_ij = 0 for all i ≠ j, and a_ii = k (where k is a constant) for all i.

3. Symmetric Matrix:
A symmetric matrix is a square matrix that is equal to its transpose. In other words, the element in the i-th row and j-th column is equal to the element in the j-th row and i-th column (a_ij = a_ji for all i and j).

4. Minor:
In linear algebra, a minor is not a type of matrix itself. Instead, a minor (specifically, the minor of an element a_ij) is the determinant of the submatrix formed by deleting the i-th row and j-th column of a larger matrix. Since a determinant is a single numerical value (scalar) and not an array/matrix, "Minor matrix" is not a standard mathematical term or type of matrix.