The systematic name for neo pentyl group is
2, 2-Dimethylpropyl
3CH3−CH3|C2|CH3−C1H2−
Shown above is the neopentyl group.when it becomes a substituent ,asthere are 2 methyl groups at c2
it is a substituted substituent.
Hence
(d)2,2-Dimethylpropyl
Note:the absence of 'ane'