For an element to be strong oxidizing agent it should itself readily undergo reduction. So among given elements we find which one will undergo reduction more spontaneously.

The given are the reduction potential of elements for which the half-cell reactions are:

We know that, higher the reduction potential of any element the more readily the given element will undergo reduction as it gives out more energy in the form of electric potential reaching a more stable state. (Here the stable state is element at lower energy)

Since among all the elements Ag has the highest potential it will undergo reduction itself more readily and thus will act as a strong oxidizing agent.