what is the sum? ABCDPlease tell me which letter is the correct answer

Matrix addition can only be performed on matrices of the same size.
This means that you can only add matrices if both matrices are m × n.
For example, you can add two or more 3 × 3, 1 × 2, or 5 × 4 matrices. You cannot add a 2 × 3 and a 3 × 2 matrix, a 4 × 4 and a 3 × 3, etc.
The number of rows and columns of all the matrices being added must exactly match.
Therefore, the answer is letter D - These two matrices cannot be added.