'Idiomatic expressions are a type of informal English that have a meaning different from the meaning of the words in the expression.'
Option D: From the options, 'ants' forms an idiomatic expression 'ants in your pants'. It means 'to be nervous and anxious'. It relates to the sentences "Why are you so restless?" and "Dan said he has bad news". Hence D is the correct option.
The other words 'moths', 'bugs' and 'bees' each do not form an idiomatic expression. They all form literal meanings in the sentence, meanings which are not quite relevant. Hence options A, B and C are incorrect.