Step 1: Recall the purpose of the Secant Formula.
The Secant Formula accounts for the combined effects of axial compression and bending caused by eccentric loading.
Step 2: Identify its application.
It is specifically applicable to
\[
\boxed{\text{Long columns under eccentric loading}.}
\]
Hence,
\[
\boxed{(D)}
\]
is the correct answer.