A pioneer of a reform movement:

The correct answer is: A. Apostle.

An apostle is a person who is sent on a mission, especially by a religious leader. In the Christian tradition, the apostles were the twelve disciples of Jesus Christ who were sent out to spread his teachings.

A apothecary is a person who prepares and sells medicines.

An apotheosis is the glorification or deification of a person.

A renegade is a person who deserts his party, cause, or principles.

In the context of the question, an apostle is the most likely answer because it is a person who is sent on a mission to reform something. The other options are not as relevant to the question.