## Core Concept
The question tests understanding of **autosomal dominant inheritance**, a pattern where a single copy of a dominant allele is enough to cause the condition. In autosomal dominant disorders, each child has a 50% chance of inheriting the mutated gene from an affected parent who is heterozygous for the condition.

## Why the Correct Answer is Right
When one parent is an **autosomal dominant heterozygote**, they have one normal allele and one disease-causing allele. The possible genotypes of their offspring according to Mendelian genetics are:
- 50% chance of inheriting the disease-causing allele (and thus being affected, either homozygous for the disease allele or, more commonly, heterozygous like the parent)
- 50% chance of inheriting the normal allele (and thus not being affected)

This 50% chance of passing the disease-causing allele to each offspring is a hallmark of autosomal dominant conditions. Therefore, the chance of an offspring being affected when one parent is an autosomal dominant heterozygote is **50%**.

## Why Each Wrong Option is Incorrect
- **Option A:** This option suggests a 0% chance, which would only be true if the condition were recessive and the parent were not a carrier or if it were not inherited in an autosomal dominant pattern.
- **Option B:** This option suggests a 25% chance, typical of autosomal recessive conditions where both parents are carriers, not applicable here.
- **Option D:** This option suggests a 100% chance, which would only be true if both parents were homozygous dominant for the condition or if the condition were X-linked dominant and the parent were male.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **autosomal dominant conditions often exhibit variable expressivity and penetrance**, meaning the severity of the condition can vary greatly between individuals, and not everyone with the disease-causing allele will show symptoms. However, the chance of passing the allele to offspring remains consistent.

## Correct Answer: .