Final answer:
An isosceles triangle is defined by having two equal sides. The question asks to identify a property of an isosceles triangle, and the correct answer is A) Two equal sides. Other triangle types, such as equilateral (three equal sides) and scalene (unequal sides), or the presence of a right angle, do not define an isosceles triangle.
Step-by-step explanation:
The question asks about the properties of an isosceles triangle measured in millimeters. An isosceles triangle is a figure with two equal sides and therefore, the correct answer is A) Two equal sides. This geometric shape lies on a plane and has three angles that add up to 180 degrees.
When considering triangles, it's essential to understand the basic properties and types of triangles. An isosceles triangle is distinct from an equilateral triangle, which has three equal sides (not applicable here), and a scalene triangle, which has unequal sides. In the context of an isosceles triangle, the equal sides are usually denoted as the 'legs', and the third side is commonly referred to as the 'base'.
Some isosceles triangles can also contain a right angle, making them isosceles right triangles, but having a right angle is not a defining characteristic of all isosceles triangles, so D) A right angle is not the correct answer for the defining property of an isosceles triangle.