It is very difficult to relax while working with their new boss because he is a hard

Explanation

The correct answer is taskmaster because it describes someone who imposes strict or demanding tasks on others, which aligns with the sentence's implication that the boss is difficult to work with due to their demanding nature. The option 'worker' does not indicate the boss's demanding nature; 'hearted' is not used properly here as it needs an adjective to describe someone's nature; and 'nut crack' is not a recognized term for describing a person.