All the listed methods are valid for solving a system of linear equations:
1. Gaussian Elimination: A systematic method to reduce the system to row echelon form.
2. Cramer's Rule: A method using determinants to solve linear equations.
3. Matrix Inversion Method: Involves finding the inverse of the coefficient matrix to solve the system.
Thus, the correct answer is: \[ \boxed{(D) \, \text{All of the above}} \]