What is the actual equation of stiffness matrix?

• A. K= ( begin{bmatrix}1 & -1 -1 & 1 end{bmatrix} )
• B. K= ( frac{AE}{l} begin{bmatrix}1 -1 end{bmatrix} )
• C. K= ( frac{AE}{l} )
• D. K= ( frac{AE}{l} begin{bmatrix}1 & -1 -1 & 1 end{bmatrix} )