Ans. a (Rat flea) (Ref. Park Textbook of PSM 22nd/pg. 712)ARTHROPODS OF MEDICAL INTEREST InsectaArachnidaCrustacea1Body divisionsHead, Thorax, AbdomenCephalothorax and abdomen (sometimes no division)Cephalothorax and abdomen2Legs3 pairs4 pairs5 pairs3Antennae1 pairNone2 pairs4WingsOne or two pairs, some winglessNoneNone5Where foundOn landOn landIn water -# Rat flea belongs to class Insecta which has 3 pairs of legs.# Option 2, 3, 4 belongs to class Arachnida which has 4 pairs of legs.# Cyclops belongs to class Crusteacea which has 5 pairs of legs.ClassIncludesClass Insecta includes# Mosquito, # Flies (houseflies, sand flies, tsetse flies, black flies) # Human lice # Fleas # Reduviid bugsClass Arachnida includes# Ticks (hard & soft tick), # Mites (trombiculid, itch mite)Class Crusteacea includes# Cyclops.