Step 1: Apply De Morgan's theorem.
According to De Morgan's law,
\[
\overline{X+Y+Z}
=
\bar{X}\,\bar{Y}\,\bar{Z}.
\]
Step 2: Substitute the variables.
Therefore,
\[
\overline{(A+B+C)}
=
\bar{A}\,\bar{B}\,\bar{C}.
\]
Hence,
\[
\boxed{\bar{A}\,\bar{B}\,\bar{C}}
\]
Therefore,
\[
\boxed{(D)}
\]
is the correct answer.