A triangle has two sides that measure 12 meters and 3 meters. Which could be the measure of the third side? A. 17 m B. 15 m C. 10 m D. 9 m

For any possible triangle, the sum of any two sides must be greater than the third side. In this case the third side must be less than 12+3=15

So only C (10) and D (9) are less than 15 and are the only possible side lengths given your options.

C. 10 m

That is the correct answer.