Answer:
It should be C.
Explanation:
For sure, test-cross is used to determine the genotype by using the two parent genotypes and cross-breeding them.
Test-cross also aims to produce "true-breeding", but I do not think this is the case 100%. True-breeding is when all 4 determined offsprings carry the same (Rr) x4. However, the reason I do not believe that it is not the case 100%, is because it relies on the parents being true themselves (RR & rr, respectively).
