The correct option is A coconut
Coconut has a thick and hard seed coat. Thin membranous seed coat is present in groundnut, gram and maize.
Ground-nut is a dry dehiscent fruit known as legume. It has a papery seed coat.
Gram and maize are indehiscent dry fruits known as grains or caryopsis. In gram, the outer seed coat is thick and strong and the inner coat is thin and membranous. In maize, the seed coat is membranous and generally fused with the fruit wall.