Concept:An eggshell is made mainly of calcium carbonate (CaCO3). A laying hen draws calcium from its feed (and its bones) to build each shell.Solution:Thin-shelled or soft-shelled eggs show that the hen is not getting enough of the mineral used to form the shell.That mineral is calcium, so the defect is mostly caused by calcium deficiency.Farmers correct it by adding oyster shell, limestone or bone meal to the layers' feed.Phosphorus also helps bone and shell formation but is not the main shell mineral; zinc and magnesium are needed only in small amounts and are not the main cause.Answer:Option D (calcium)