Which number is in all the geometrical figures?

The correct answer is Option (3) → 4

A number belongs to a figure when it lies inside that figure's outline. The number asked for must lie inside the triangle, the rectangle and the circle at the same time, that is, in the region common to all three.

Only 4 sits where all three figures overlap. The others each miss at least one figure: 5 is outside the circle, 3 is outside the rectangle, and 8 is inside the circle only.