Final answer:
According to Adler's birth order theory, a competitive and ambitious person is most likely to be a firstborn, as they may strive to overachieve due to their role in the family.
Step-by-step explanation:
In the context of Adler's idea of birth order and personality, a person who is competitive and ambitious is most likely to be a: a) Firstborn. Adler proposed that older siblings might become overachievers to compensate for the divided attention from their parents after the birth of subsequent children. This competitive nature and drive for achievement are often associated with firstborns as a result of their early years as the sole focus of their parents' attention.