Which of the following does not belong to the first transition series?

Ag

The first transition series consists of the elements from scandium \((Sc)\) to zinc \((Zn)\) in the periodic table. These elements have their 3d orbitals partially or completely filled.

So, the answer is 4. Ag (silver) does not belong to the first transition series. Silver belongs to the first main group or alkali metals. Its electronic configuration is \([Kr] 4d^{10} 5s^1\), which shows that its \(3d\) orbitals are not partially or completely filled.