This question was previously asked in

CTET Jan 2021 Paper 2 (Maths & Science) (English-I/ Hindi-II)

Option 3 : a necessary but not a sufficient condition.

A **triangle** is a two-dimensional figure which has three sides and the sum of whose angles is 180 degrees.

__Key Points__

There are** certain conditions **for a figure to be triangle as stated below:

**There are three sides of a triangle, which is necessary. Having three sides does not guarantee that the figure is a triangle.**- The sum of lengths of two sides should always be greater than the third side.
- The difference of length of two sides should always be less than the third side.

Hence, we conclude that for a given figure to be a triangle, the condition that it is a union of three segments is a necessary but not a sufficient condition.