The plural that is not formed correctly is d. roof = rooves. The modern version of the plural of roof is "roofs". Rooves was used in the UK and New England during the nineteenth century. It is still occasionally seen with other plurals such as hoof = hooves, but it is not common. The English language is full of rules with exceptions to the rule. For example, the plural of hoof is hooves, but the plural of roof is not rooves. It is roofs.