**Correct option is (C) 20**

In parallelogram ABCD, AC is a diagonal.

\(\because\) area of \(\triangle ABC\) = area of \(\triangle ACD\)

Also, area of \(\triangle ABC\) + area of \(\triangle ACD\) = area of rectangle ABCD.

\(\therefore\) Area of rectangle ABCD \(=2\times\) Area of \(\triangle ABC\)

\(=2\times10\,cm^2\)

\(=20\,cm^2\)