Is there a shortcut key in ModelSim so that part of the code I wrote will be turned to a comment:?
Although I did not manage to find such a shortcut in modelsim, I found a way to create one. Follow these steps :

1) In modelsim, goto Window -> Keyboard Shortcuts. A dialogue will popup.

2) On the top, under Shortcut type, select "Window Specific" .

3) In the text box area, enter your desired key combination. Note that some shortcuts are reserved and cant be changed (including your preferred ctrl+k).

4) In the 'Select Command From' Radio buttons, select "Popup or Pull-Down Menu".

5) In the 'Menu Items' , select "Source -> More -> Comment Selected" .

6) Press Ok.

Voila ! You now have your shortcut ready.