When does write acknowledgement happen?

A developer of a business-critical application is concerned about system reliability when it comes to a IO resistance.

An architect needs to explain the FlashBlade write mechanism and address three main concerns:

– Data location when FlashBlade acknowledge a write

– Number of created data copies before FlashBlade acknowledges a write

– Blade failure resilience

When does write acknowledgement happen?
A . When data is written onto Flash.
B. When data is written NvRAM on three different blades.
C. When data is written NvRAM on two different blades.
D. When data is written NvRAM on a super capacitor.