Two bodies begin to free fall from same height at a time interval of N seconds, If the vertical seperation between them is 1m after n seconds from the start of 1st body, then n equals

Question

$$(a)\;\sqrt {nN} \quad (b)\;\frac{1}{gN} \quad (c)\;\frac{1}{gN}+\frac{N}{2}\quad (d)\frac{1}{gN}-\frac{N}{4}$$