Which phrase best describes someone who is fully occupied with their tasks?

Explanation

Option A, 'ins and outs,' refers to details or intricacies. Option B, 'up to the neck,' means being deeply involved but is less common. Option C, 'up to the hilt,' correctly means being completely engaged or overwhelmed with something. Option D, 'kith and kin,' relates to relatives and close friends, not workload.