To identify the wrong match, we evaluate the molecular geometry of each species based on VSEPR theory and hybridisation as described in the sources:

1. $PCl_{5}$ (Phosphorus pentachloride): Phosphorus has five valence electrons and forms five sigma bonds with chlorine atoms ($AB_{5}$ type). This results in $sp^{3}d$ hybridisation and a trigonal bipyramidal geometry . List-II matches it with 'Trigonal planar', which is incorrect.
2. $SF_{6}$ (Sulphur hexafluoride): Sulphur has six valence electrons and forms six sigma bonds with fluorine atoms ($AB_{6}$ type). This results in $sp^{3}d^{2}$ hybridisation and an octahedral geometry . This is a correct match.
3. $BeCl_{2}$ (Beryllium chloride): Beryllium has two valence electrons and forms two sigma bonds ($AB_{2}$ type). This results in $sp$ hybridisation and a linear geometry . This is a correct match.
4. $NH_{3}$ (Ammonia): Nitrogen has five valence electrons, forms three sigma bonds, and has one lone pair ($AB_{3}E$ type). This results in $sp^{3}$ hybridisation and a trigonal pyramidal geometry . This is a correct match.

Since pair (a) is the only incorrect match, the correct choice is 'a'.