Correct Answer - Option 3 : 47

__Concept:__

\(\rm Mean\ of\ 'n'\ observations=\dfrac{Sum\ of\ observations}{n}\).

__Calculation:__

Let's say that the third number is x.

So, Numbers are 4, 12 and x

Here n = 3

As we know, \(\rm Mean\ of\ 'n'\ observations=\dfrac{Sum\ of\ observations}{n}\)

∴ \(\rm 21=\dfrac{4+12+\rm x}{3}\)

⇒ x + 16 = 63

⇒ x = 47.