Final answer:
Trimethoprim-sulfamethoxazole (TMP/SMZ) is contraindicated in the third trimester of pregnancy due to the risk of kernicterus in the newborn.
Step-by-step explanation:
The antibiotic for cystitis that is contraindicated in the third trimester of pregnancy is C. Trimethoprim-sulfamethoxazole (TMP/SMZ). During the third trimester, the use of TMP/SMZ can cause kernicterus in the newborn, a condition that affects the brain due to a high level of bilirubin. For this reason, TMP/SMZ is avoided in late pregnancy. While other options such as Ciprofloxacin, Nitrofurantoin, and Ampicillin have various degrees of safety profiles during pregnancy, TMP/SMZ has a specific contraindication at this stage.