**Core Concept**
The question is testing the understanding of the clinical staging and prognosis of cervical cancer, specifically the incidence of lymph node metastasis in stage II carcinoma cervix. This involves knowledge of the FIGO staging system and the prognostic factors associated with cervical cancer.

**Why the Correct Answer is Right**
In stage II carcinoma cervix, the tumor has grown beyond the cervix and has reached the upper vagina, but has not extended to the pelvic wall. The incidence of lymph node metastasis in stage II cervical cancer is relatively high, ranging from 10-20%. This is because the tumor has already invaded the deeper tissues and is more likely to spread to the lymph nodes. The presence of lymph node metastasis is an important prognostic factor, as it can significantly affect the treatment plan and outcome.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because stage I cervical cancer is typically confined to the cervix and has a lower incidence of lymph node metastasis.

**Option B:** This option is incorrect because stage III cervical cancer is more advanced, with tumor invasion of the pelvic wall and/or involvement of the lower third of the vagina and/or parametrium, resulting in a higher incidence of lymph node metastasis.

**Option C:** This option is incorrect because stage IV cervical cancer is the most advanced stage, with tumor invasion of the bladder or rectum, and/or distant metastasis, resulting in a very high incidence of lymph node metastasis.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the incidence of lymph node metastasis in cervical cancer is directly related to the stage and grade of the tumor, as well as the presence of symptoms and patient performance status.

**Correct Answer:** B. 15%