**Core Concept**
Otitis externa, also known as swimmer's ear, is a type of external ear infection characterized by inflammation of the outer ear canal. It often occurs after exposure to water, particularly in individuals who swim frequently. The condition is typically caused by bacterial or fungal overgrowth in the ear canal.

**Why the Correct Answer is Right**
The recommended method for preventing recurrent otitis externa in this young boy is to use ear drops containing **acetic acid**. Acetic acid has antimicrobial properties that help to reduce the pH of the ear canal, creating an environment that is less conducive to bacterial or fungal growth. This, in turn, reduces the risk of infection. Additionally, acetic acid can help to dry the ear canal, which is also beneficial in preventing otitis externa.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is not recommended because antibiotics are not effective in preventing otitis externa unless there is a confirmed bacterial infection. In this scenario, the boy is experiencing recurrent bouts, suggesting a predisposition to infection rather than an active bacterial infection.

**Option B:**
This option is not recommended because drying the ear canal with a towel may push earwax further into the ear canal, potentially exacerbating the condition.

**Option C:**
This option is not recommended because using a cotton swab to clean the ear canal can push earwax further into the ear canal, potentially causing damage to the eardrum or ear canal.

**Clinical Pearl / High-Yield Fact**
To prevent otitis externa, it's essential to dry the ear canal thoroughly after swimming by gently tilting the head to the side and allowing the water to drain out. Avoid using cotton swabs or inserting objects into the ear canal.

**Correct Answer: C.**