An incomplete for

Element D

Atomic size goes on increasing down a group because:

i. Down a group, the atomic number increases one by one.

ii. The outer electrons are adding in a new valence shell.

iii. Therefore, the distance between the outermost electron (valence electron) and the nucleus is also increasing.

iv. Therefore, the size of the atom increases with increase in atomic number.

Number of shells are increasing down a group

Thus, D has the biggest atom in group 1.