**Core Concept**
The question refers to the Schick test, a method used to assess the susceptibility of an individual to diphtheria toxin. The test involves injecting a small amount of diphtheria toxin into the skin, and the person's ability to form an immune response against it is measured by their reaction to the toxin.

**Why the Correct Answer is Right**
The correct answer is based on the principle that a child becomes immune to diphtheria toxin after receiving the diphtheria toxoid vaccine. The vaccine stimulates the production of antibodies against the toxin, which can be detected by the Schick test. A positive Schick test result indicates that the individual is susceptible to diphtheria, while a negative result shows that they have developed immunity. The age at which a child can hold the film in the Schick test is a critical factor in determining their susceptibility to diphtheria.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the correct age at which a child can hold the film.
**Option B:** This option is incorrect because it does not provide a specific age for holding the film, making it an ambiguous choice.
**Option C:** This option is incorrect because it is not the correct age at which a child can hold the film in the Schick test.

**Clinical Pearl / High-Yield Fact**
The Schick test is a crucial diagnostic tool for identifying individuals who are susceptible to diphtheria. It is essential to administer the diphtheria toxoid vaccine to children at the correct age to prevent diphtheria and ensure they develop immunity against the toxin.

**Correct Answer:** 3 years.