Answer:
C. 17 units
Explanation:
You want the hypotenuse of a right triangle with legs 8 and 15.
Pythagorean theorem
The Pythagorean theorem tells you the relation between the sides of a right triangle is ...
c² = a² +b² . . . . . . where c is the hypotenuse, and a, b are the legs
Application
c² = 8² +15² = 64 +225 = 289
c = √289 = 17 . . . . units
The missing side length is 17 units.
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Additional comment
A set of integers that satisfies the Pythagorean theorem is called a "Pythagorean triple." Perhaps the first one you run across is {3, 4, 5}. Other triples commonly seen are {5, 12, 13}, and {7, 24, 25}. This one, {8, 15, 17} also shows up in algebra and trig problems. It can be worthwhile to remember a few of these.
Their multiples are also seen. For example, {6, 8, 10} is the {3, 4, 5} triple times 2.