The official answer key says option B but we believe the right option is C.

The author does not say there is no difference in accuracy between a grandfather clock and an atomic clock. In fact he clearly states the opposite: '...a large, inefficient grandfather clock isn't more precise than an atomic clock.' So, option C is not an argument made in the passage.

On the other hand, option B is an argument the author makes in the first paragraph: '..the researchers found that as they increased the clock's accuracy, the heat produced in the system grew, increasing the entropy of its surroundings by jostling nearby particles . . . "If a clock is more accurate, you are paying for it somehow," says Ares.'

Options A and D are easily verified to be true, based on the lines 'The relationship that the researchers found is a limit on the accuracy of a clock, so it doesn't mean that a clock that creates the most possible entropy would be maximally accurate', and 'Understanding this relationship could be helpful for designing clocks in the future, particularly those used in quantum computers and other devices where both accuracy and temperature are crucial...'.