Hint: Some genera of mollusc have branched projection cerata on their upper body surface as lateral outgrowth which helps in respiration. Except respiration cerata also function for defence mechanism and for trapping its food.
Invertebrate nudibranchs which are scientifically called as Nudibranchia are shell-less molluscs which have jelly type body structure in different colour forms. Nudibranchia means organism with naked gills, these gills called cerata which are secondary gills present on the upper surface of body function for gas exchange. This gill formed after the disappearance of the mantle cavity and original gills. Nudibranchs also use their cerata for defence mechanism by secreting cnidarians stinging cells, these cells mature in cerata and form nematocysts. In some Nudibranch nematocysts are absent on cerata, these nudibranchs drop their one cerata when they are in danger, dropped cerata shows movement for some time by which predator got distracted and nudibranch escape out.
Diverticulum present inside cerata which are naked due to the transparent body structure of nudibranch. Diverticulum acts as a digestive gland, the colour of nudibranch also depends upon the food intake.
Additional Information: Squids come under class cephalopods of phylum Mollusca. Squids have long slender tentacles with toothed horny rings in eight tentacles and two tentacles teeth are absent, these tentacles hold the food and break it into small pieces for ingestion.
Phylum Echinodermata consist of species Cucumaria miniata which have retractile tentacles on the oral surface. These tentacles help to trap small organisms and detritus present in water.
So, the correct answer is ‘Secondary anal gills’.
Note: The nudibranch is hermaphrodite animals, they lay an egg in the form of a long string protected by a mucus covering. The Indopacific ocean is home to many species of nudibranch. Nudibranchs show shells on their body surface at the larval stage which get exhausted by ocean acidification that's why in an adult form they are shell-less.