**Core Concept**
Vaginal agenesis, also known as Mayer-Rokitansky-Küster-Hauser (MRKH) syndrome, is a congenital anomaly characterized by the absence or underdevelopment of the vagina and uterus. The ideal age for repair of vaginal agenesis depends on various factors, including the patient's physical and emotional maturity.

**Why the Correct Answer is Right**
The correct answer, "Before marriage," is based on the principle that vaginal agenesis repair should be performed before the patient undergoes significant physical and emotional changes associated with marriage and childbearing. This allows for optimal results and reduces the risk of complications. Additionally, performing the repair before marriage can help alleviate psychological distress and improve the patient's overall quality of life.

**Why Each Wrong Option is Incorrect**
**Option A:** Repairing vaginal agenesis at 6 months is not ideal due to the patient's immature physical and emotional state. At this age, the patient may not be able to fully comprehend the procedure or its implications.

**Option B:** Repairing vaginal agenesis at 3 years is not suitable, as the patient's physical and emotional development is still in its early stages. Moreover, the child may not be able to communicate effectively or understand the procedure.

**Option C:** Performing the repair at puberty may not be the best option, as the patient may experience significant physical and emotional changes that can affect the outcome of the procedure.

**Clinical Pearl / High-Yield Fact**
It is essential to note that vaginal agenesis repair should be performed by an experienced gynecologist or urogynecologist to ensure optimal results and minimize the risk of complications. Additionally, patients with vaginal agenesis may require additional interventions, such as dilation or vaginoplasty, to achieve satisfactory vaginal length and function.

**✓ Correct Answer: D. Before marriage. Before marriage is the ideal time for repair of vaginal agenesis to ensure optimal results and minimize complications.**