Which of the following keys of keyboard are called modifier keys? (a) Ctrl (b)…

A modifier key is a keyboard key that has no fixed effect of its own; it produces an action only when held down together with another key (or a mouse click), and its role is to change -- "modify" -- what that other key or click does. A key that carries out a complete action entirely by itself (such as moving the text cursor) is a navigation or function key, not a modifier key.

Checking each key named in the question against this definition:

• Ctrl (Control): has no effect pressed alone; combined with a letter key it triggers a command, e.g. Ctrl+C (copy), Ctrl+V (paste). It is a modifier key.

• Shift: combined with a letter key it produces a capital letter, and combined with a mouse click it extends a selection. It is a modifier key.

• End: pressed by itself, it moves the cursor to the end of the current line or document -- it needs no companion key to act. It is a navigation key, not a modifier key.

• Alt (Alternate): combined with another key it triggers an alternate command, e.g. Alt+Tab (switch windows), Alt+F4 (close window). It is a modifier key.

Ctrl, Shift, and Alt each satisfy the modifier-key definition because they act only in combination with another key or click, while End acts completely on its own and so fails that definition. The set of modifier keys among those listed is therefore Ctrl, Shift, and Alt.