The use of tasks to promote mathematical reasoning (MR) in teaching practice is essential to meet curricular goals. However, that practice is often a huge challenge for teachers, and particularly for prospective teachers and thus it is essential to highlight it as a goal for initial teacher education. This study focuses on preservice mathematics teachers' (PTs) knowledge about the potential of mathematical tasks to promote students' MR, in a teacher education course. Results show that PTs were able to justify their option for a mathematical task with potential to promote students' MR, and through its implementation in one 8th grade classroom they have deepen their knowledge and gave greater meaning to task design principles and acknowledging their students' knowledge. Thus, the activity of selecting and adapting a task, although less demanding than the design of a new task, can still provide PT with important reflection and knowledge about its potential to promote students' MR. The study stresses the relevance for initial teacher education of considering four domains associated with the recognition of the potential of tasks to promote MR.