The correct answer is C) tap the holes without cutting oil. When hand tapping holes in cast iron, it is generally recommended to tap the holes without using any cutting oil. Cast iron is a brittle material, and the use of cutting oil can cause the cast iron to crack or break during the tapping process. Instead, the tapping should be done slowly and carefully, allowing the tap to cut its own way through the cast iron without the added lubrication of cutting oil. The other options, such as starting with a plug tap (A) or chamfering the edges (B), are not necessary for this specific task, and flooding the tap with mineral oil (D) could potentially lead to the undesirable cracking or breaking of the cast iron.