The correct option is A better go to bed
Before choosing any option, we need to examine all of them within the context given. Here, the context indicates that one has a terrible cold.
So, option A gives the only sensible advice of 'go to bed'. This indicates that one should take some rest and concurs with the context.
Option B is incorrect as it advises not to go to a doctor when one is sick. This does not make sense.
Option C is incorrect as it advises to go out.
Option D advises to work harder when one is sick.
Thus, only option A is correct.