Some claim that utilitarianism demands more calculation than we are capable of. john stuart mill responded to this by claiming that a. with training, people can learn to make such complicated calculations well. b. this is equally a problem for every moral theory. c. calculations are often unnecessary because we can usually rely on common wisdom. d. this explains why no one has moral knowledge.

C) Calculations are often unnecessary because we can usually rely on common wisdom.

Explanation: According to the ethical theory of John Stuart Mill about utilitarianism is that morality of actions are used for defining the result or outcome about the happiness . Calculations are thus not a requirement for the utilitarianism.

Other options are incorrect because training is not required for simplification of calculation.  It is not an issue for every theory and neither is the reason for low knowledge about morals.Thus, the correct option is option(C)