What is the transpose of a matrix?

The transpose of a matrix is the matrix obtained by interchanging rows and columns.
The transpose of a matrix is the matrix obtained by adding the elements of the main diagonal.
The transpose of a matrix is the matrix obtained by multiplying all elements by a scalar.
The transpose of a matrix is the matrix obtained by subtracting the elements of the main diagonal.
a) I is correct.
b) II is correct.
c) I and III are correct.
d) None of the above are correct.