The correct option is B NH−
Carbocation is an electron deficient species. When an electron-donating group (+I group) is attached to a electron deficient species, it stabilizes the positive charge and makes the carbocation stable.
Among the given options, only −CR3 and −NH− groups show the +I effect. The electron-donating power of −NH− group is higher than that of −CR3 group due to the formal negative charge.
Hence, the most preferred group in place of X will be NH−.