Which key, when pressed, display the access keys in MS Excel ?

The correct answer is Option (4) → [Alt]

Pressing [Alt] displays the access keys: a letter appears over each tab and command of the ribbon, and pressing that letter next opens the tab or runs the command, so the ribbon can be used from the keyboard without a mouse. [Ctrl] and [Shift] are used with other keys for shortcuts and selection, and [Esc] cancels an entry or closes a menu; none of them shows the access keys.