**Core Concept**
Transitional cell carcinoma (TCC), also known as urothelial carcinoma, is a type of cancer that arises from the lining of the urinary system, including the bladder, ureters, and renal pelvis. The management of TCC depends on the tumor's stage, grade, and histological type.

**Why the Correct Answer is Right**
Given that the patient has a solitary papillary tumor with no muscle invasion, the tumor is classified as pT1a, which is a non-muscle-invasive bladder cancer (NMIBC). The next best step in management is intravesical immunotherapy or chemotherapy to prevent recurrence and progression. This involves instilling a medication directly into the bladder through a catheter to target the cancer cells. In this case, intravesical Bacillus Calmette-Guérin (BCG) is the preferred treatment due to its efficacy in reducing recurrence rates and preventing progression to muscle-invasive disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Radical cystectomy is typically reserved for muscle-invasive bladder cancer (MIBC) or high-risk NMIBC, not for a solitary papillary tumor with no muscle invasion.

**Option B:** Radiation therapy is not the first-line treatment for NMIBC, although it may be considered in certain cases or as part of a multimodal approach.

**Option C:** Chemotherapy is not typically used as a first-line treatment for NMIBC, but rather as an adjuvant therapy or in cases of recurrence or progression.

**Clinical Pearl / High-Yield Fact**
Intravesical BCG is the preferred treatment for high-risk NMIBC due to its ability to induce a systemic immune response against cancer cells, reducing recurrence rates and preventing progression to MIBC.

**Correct Answer:** C. Intravesical BCG.