The unconscious prototype that people have of a family leads them to overestimate the probability that a family will have exactly a daughter, son, and daughter. This reasoning error illustrates:

Answer:The representativeness heuristic.

Explanation:

The representativeness heuristic is when we use a shortcut to make judgements or decisions. In representativeness heuristic we tend to overestimate the likelihood of a situation based on our pre-existing infomation, which is referred to as our prototype.

For example in this given scenario people tend to estimate future births based on the observed existing births within families.