Step 1: Identify the end condition.
The column is
- Fixed at one end
- Free at the other end
This is a cantilever column.
Step 2: Recall the effective length factor.
For a cantilever column,
\[
K=2.
\]
Therefore,
\[
L_e=KL=2L.
\]
Hence,
\[
\boxed{2L}
\]
is the correct answer.
Therefore,
\[
\boxed{(D)}
\]
is the correct answer.