**Core Concept**
The question tests the understanding of **sex-linked traits**, which are inherited in a pattern linked to the sex chromosomes. These traits are typically seen on the X chromosome and can be dominant or recessive.

**Why the Correct Answer is Right**
To rule out a sex-linked trait, one would look for an observation that contradicts the expected inheritance pattern of these traits. Since the correct answer options are not provided, a general explanation is that any scenario where the trait does not follow the typical X-linked pattern (e.g., a father passing the trait to his son) would suggest it is not sex-linked.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the specific details, we cannot directly address why this option is incorrect, but generally, any option that aligns with the typical pattern of sex-linked inheritance would not rule out the trait.
**Option B:** Similarly, without specifics, we can say that if this option supports the sex-linked pattern, it would not be correct.
**Option C:** This option, if it showed a clear deviation from the expected sex-linked inheritance, could potentially be the correct answer, but without details, we cannot confirm.
**Option D:** If this option fits the pattern of sex-linked traits, it would be incorrect as it would not rule out the trait.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **sex-linked traits** rarely pass from father to son, as fathers give their sons a Y chromosome, not an X chromosome. If a trait is seen being passed from father to son, it strongly suggests the trait is not sex-linked.

**Correct Answer:** Correct Answer: C. An affected male having an affected son.