$\textbf{Option A :}$ Full Subtractor using NAND / NOR
$\textbf{Option B :}$ Half Adder using NAND / NOR
$\textbf{Option C :}$ $\text{NAND gate needs less power to startup}$ refer
$\textbf{Option D:}$
$A \oplus B = Σ_{m}\ (1,2)$
$A \oplus B \oplus C = Σ_{m}\ (1,2, 4, 7)$
$ A \oplus B \oplus C \oplus D = Σ_{m}\ (1,2, 4, 7, 8, 11, 13, 14)$
$\text{ For 2 vars, no of minterms = 2 = }$ $2^{2-1}$
$\text{ For 3 vars, no of minterms = 4 = }$ $2^{3-1}$
$\text{ For 4 vars, no of minterms = 8 = }$ $2^{4-1}$
$\text{ … … … … … … … … … }$
$\text{ For n vars, no of minterms = }$ $2^{n-1}$
$\underline {\text{Intuition}}:$ $\text{for 3 var we got minterms 1,2,4,7 so during 4 var after 7, if we subtract current minterms,}$
$\text{we will get remaining minterms upto 15 as for 4 vars the highest minterm is 15}$
$\text{ so, 15 – 1 = 14, 15 – 2 = 13, 15 – 4 = 11, 15 – 7 = 8} $
$\text{ Similarly, for 5 var we need to subtract from 31 and so on ...}$
