B is false. You cannot square a 3x1 matrix.
In order to multiply matrices, the number of columns of the first matrix must match the number of rows of the second matrix. To square a 3x1 matrix, you would be multiplying a 3x1 by a 3x1; the number of columns in the first one is 1, but the number of rows of the second matrix is 3. They are not compatible.