(a)PCl5 act as an oxidizing agent because the oxidation state of Phosphorous in PCl5 is +5. It has 2 more oxidation states than +5 ie +3 and -3.
+3 oxidation state is more stable than +5. P gets reduced from +5 to +3. It can not act as a reducing agent as it cannot donate electrons.
(b) Dinitrogen is a combination of two nitrogen it has a triple bond and bonded via a covalent bond. The inertness is due to the high bond enthalpy because it requires a higher amount of energy to break the bond of N2 which is a combination of 2 pi and 1 sigma bond