The correct option is A Nautilus
Mantle, foot and shell are the characters of the Mollusca. Of the given options, only Nautilus is marine mollusc which belongs to the cephalopod family Nautilidae. Echinus belongs to the phylum Echinodermata. Limus (horseshoe crab) belongs to the phylum Arthropoda. Euplectella belongs to the phylum Porifera. So, the correct answer is option A.