Which is the best definition of an isosceles triangle

Option 1 is correct.

Step-by-step explanation:

To find: Correct definition of Isosceles triangle

Isosceles triangles are the triangle whose 2 side are equal.

Option 1:

A triangle that has at least two congruent side.

congruent means equal.

Here, triangle has at least 2 equal side which is of isosceles triangle.

Thus, this option is correct.

Option 2:

A triangle in which no 2 sides are congruent.

Here it is said that no two sides are equal this contradict the fact of isosceles having two equal side.

Thus this option incorrect.

Option 3:

A triangle that has three congruent sides.

Here All 3 sides of triangle are equal. this implies triangle is an equilateral triangle not the isosceles triangle.

Thus, this option is incorrect.

Therefore, Option 1 is correct.