Balaenoptera, Camelus, Delphinus, Macropus, Ornithorhynchus, Pteropus - Choose the odd one out with reason

Question

( a ) Ornithorhynchus is a member of class - Aves
( b ) Ornithorhynchus is an Oviparous mammal
( c ) Balaenoptera is a pteridophyte
( d ) Pteropus is not a mammal