Solving a is the process of calculating unknown side lengths or angle measures of a triangle if certain of the side lengths and/or angle measures are known.

a. square
b. polygon
c. triangle
d. problem

Triangle

Step-by-step explanation:

A triangle is a closed figure bounded by straight lines.  

A triangle has three sides, three angles and three vertices.

Normally any triangle is uniquely determined by any 3 independent measures,

It may be either 2 angles one side, or two sides, one included angle, or any other detail.  

By having 3 known dimensions of a triangle, to find out other unknown is known as solving a triangle.

Hence right answer is

Solving a _triangle is the process of calculating unknown side lengths or angle measures of a triangle if certain of the side lengths and/or angle measures are known.