Which of the following does not belong to 3d-series of transition elements?

Silver 

The answer is 3. Silver. Silver is not a transition element because it does not have a partially filled d-orbital. Transition elements are defined as elements that have a partially filled d-orbital in their ground state or in any of their common oxidation states. Silver has a completely filled d-orbital, so it is not a transition element.

The other three elements, titanium, iron, and vanadium, all have partially filled d-orbitals, so they are all transition elements.

Here is a table of the 3d-series of transition elements: