Step 1: Use the average output voltage formula.
For a full-wave rectifier,
\[
V_{\text{avg}}
=
\frac{2V_m}{\pi},
\]
where \(V_m\) is the peak voltage.
Given,
\[
V_m=100\,\text{V}.
\]
Step 2: Calculate the average voltage.
\[
V_{\text{avg}}
=
\frac{2\times100}{\pi}
=
\frac{200}{3.1416}
\approx63.7\,\text{V}.
\]
Hence,
\[
\boxed{63.7\,\text{V}}
\]
Therefore,
\[
\boxed{(C)}
\]
is the correct answer.