A family has four girls. What is the chance that a fifth child is a girl?

Each birth is an independent event. This means that the gender of previous children has no bearing on the gender of the next child. The probability of having a boy or a girl resets with each new pregnancy.

In human biology, the chance of having a girl is approximately equal to the chance of having a boy. Therefore, for any given birth, the probability of having a girl is $\frac{1}{2}$ or $50\%$.

Since the gender of the fifth child is an independent event, the fact that the family already has four girls does not change the probability for the fifth child. The chance that the fifth child is a girl remains $50\%$.

The final answer is $\text{B}$