What day would Activity C complete if the duration of Activity B was reduced to...

 If the duration of Activity B is reduced to zero, the completion of Activity C is determined solely by its own duration and predecessor relationships.

 Without knowing the explicit network logic, it is inferred that reducing Activity B to zero does not affect Activity C’s finish on Day 50, as its relationship with other activities remains intact.

 References: PSP Study Guide, Section 2.2.4 – Relationships and Calculations, highlights how relationships and durations impact completion dates.