Okay, let's tackle this question. The user is asking about the probability of a son being color blind when the mother is normal but her father was color blind, and the father is normal. So, color blindness is an X-linked recessive trait, right? Let me recall the inheritance patterns.

First, the mother is a carrier because her father was color blind. Since color blindness is X-linked recessive, a female needs two copies to show the trait, but males only need one. The mother would have one normal X and one X with the color blindness gene. The father is normal, so his X is normal. When they have a son, the son gets his X from the mother and Y from the father. So the mother is a carrier (X^C X^c), and the father is X^C Y.

For each son, there's a 50% chance of getting the X^C or X^c from the mother. Since the father can only give Y to the son, the son's X comes from the mother. If the son gets the X^c, he'll be color blind. If he gets X^C, he's normal. So the chance is 50%.

Now, looking at the options, the correct answer would be 50%. Let me check the distractors. If the options included 25%, that's when the mother is a carrier and the father is affected, but in this case, the father is normal. Another common mistake is thinking the mother is affected (homozygous) which would make all sons color blind, but here she's a carrier. So the answer is 50%.

**Core Concept**
Color blindness is an X-linked recessive disorder. A female carrier (X^C X^c) has a 50% chance of passing the affected X chromosome (X^c) to her son, who inherits the Y chromosome from the father.

**Why the Correct Answer is Right**
The mother is a carrier (X^C X^c) because her father was color blind (X^c Y), and the father is normal (X^C Y). Each son has a 50% chance of inheriting the X^c chromosome from the mother and the Y chromosome from the father, resulting in color blindness (X^c Y). Sons who inherit the X^C chromosome from the mother will be normal (X^C Y).

**Why Each Wrong Option is Incorrect**
**Option A:** Suggests 0% chance, which is incorrect because the mother is a carrier.
**Option B:** Implies a 25% chance, which would occur if both parents were carriers or if the father was affected.
**Option C:** Suggests 75% or 100% chance, which is incorrect as the father contributes only a Y chromosome to sons.

**Clinical Pearl / High-Yield Fact**
For X-linked recessive disorders:
- Carrier females (X^C X^c) have a 50% chance of passing the affected X to sons.
- All daughters of an affected father (X^c Y) will be carriers, but sons are unaffected by the father’s X.

**Correct Answer: C. 50%**

✓ Correct Answer: B. 50%