The "elastic clause" gives congress implied power to act in areas not specifically expressed in the constitution at the time it was written. congress is exercising its power under the "elastic clause" when it

The correct answer is:
When it legislates or makes laws. The "elastic clause" tells Congress that it can make all laws necessary to carry out its duties. The clause demonstrates a recognition by the framers of the constitution that issues would come up that were not covered in the Constitution. As such, they gave Congress the power to legislate.

This link explains it