From your screenshot it appears you have ReSharper installed.

Depending on the key binding options you chose when you installed it, some of your standard shortcuts may now be redirected to ReSharper commands. It's worth checking, for example Ctrl+E, C is used by R# for the code cleanup dialog.

You can also add the toolbar in Visual Studio to have the buttons available.

View > Toolbars > Text Editor

This is how I did it,

Menu Tools → Options on the Environment → Keyboard window

One can alter the default shortcuts following the below steps

• Select Edit.CommentSelection in the listbox
• Click on "Remove" button
• Select "Text Editor" option in the dropdown under "Use new shortcut in:"
• Press your own shortcut in the textbox under "Press shortcut keys:" Example: Pressing Ctrl+E and then C will give you Ctrl+E, C
• Click on "Assign" button
• Repeat the same for Edit.UnCommentSelection (Ctrl+E, U)

Use the keys CtrlK,C to comment out the line and CtrlK,U to uncomment the line.

"commentLine" is the name of function you are looking for. This function coment and uncoment with the same keybinding

I went to menu: Tools → Options.

Environment → Keyboard.

Show command containing and searched: comment

I changed Edit.CommentSelection and assigned Ctrl+/ for commenting.

And I left Ctrl+K then U for the Edit.UncommentSelection.

These could be tweaked to the user's preference as to what key they would prefer for commenting/uncommenting.