The atomic radius of Ag is closed to :

Solution

Chemistry ma’am · 3 steps

Silver and gold sit in the same group, and the lanthanide contraction makes them almost the same size.

1. Normal group trend. Going down a group, a new shell is added, so the atomic radius usually grows. That holds from Cu (period 4) to Ag (period 5).
2. Lanthanide contraction. Between Ag and Au, the 14 lanthanides fill the 4f subshell. The 4f electrons shield the nucleus poorly, so the effective nuclear charge rises and pulls the outer shell inward. This cancels the size gain from the extra shell.

   Element	Position	Atomic radius (pm)
   Ag	Group 11, period 5	144
   Au	Group 11, period 6	144
   Cu	Group 11, period 4	128
   Ni	Group 10, period 4	125
   Hg	Group 12, period 6	about 150

3. So. The atomic radius of Ag is closest to that of Au, option (a).

The idea to keep. In the d-block, the 5d element of a group has nearly the same radius as the 4d element above it (Zr–Hf, Nb–Ta, Ag–Au). That is the lanthanide contraction at work.

Answer from the NTA final answer key. Solution by PrepWiser.