Answer
Column 4 or 3
Work Step by Step
For $\begin{bmatrix}
12&-7&11&-9&5\\
-9&4&-8&7&-3\\
-6&11&-7&3&-9\\
4&-6&10&-5&12
\end{bmatrix}$ to span $R^4$, each column must be a pivot column. By finding the row reduced echelon form, we can see that column 4 is not a pivot column, therefore it can be removed.
$\begin{bmatrix}
1&0&0&-\frac{10}{21}&0\\
0&1&0&-\frac{25}{84}&0\\
0&0&1&-\frac{41}{84}&0\\
0&0&0&0&1
\end{bmatrix}$
However, it is also possible to delete column 3
