The probability of having a girl as the seventh child is 50%.
The sex of a child is determined by the combination of chromosomes from the parents. In humans, females have two X chromosomes (XX), and males have one X and one Y chromosome (XY).
Each child's gender is independent of the previous child's gender. Therefore, the fact that a couple has had six boys in a row does not affect the probability of having a girl as their seventh child. The probability of having a girl or a boy is always approximately 50% for each pregnancy.
Hence, the probability of having a girl as their seventh child is 50%. Therefore, the answer is C) 50%.