How many meiotic division are required for the formation of 400 functional megaspores?

The correct option is A 400
Megaspore mother cell (MMC) undergoes one meiotic division to form four haploid cells (called megaspores) and the process is known as megasporogenesis.
Of the four megaspores, 3 are non-functional and degenerates. Hence one meiotic division produces one functional megaspore.
So, for 400 functional megaspores 400 meiotic divisions are required.