The correct answer is D) heat the copper gasket cherry red and quench it in water. To anneal a copper gasket, you need to heat the gasket to a cherry red temperature and then quench it in water. This process softens the copper by relieving internal stresses and restoring the metal's original ductility. Quenching the hot gasket in water rapidly cools it, which is the key step in the annealing process. The other options are incorrect because: A) Letting the gasket cool slowly in air would not provide the rapid cooling needed for effective annealing. B) Quenching in oil would not provide the same level of rapid cooling as quenching in water. C) Carbonizing the gasket would involve a different heat treatment process that is not appropriate for annealing.