The correct answer is D

Explanation

Machine X, working at a constant rate, can produce x bolts per hour, and machine Y, working at a constant rate, can produce x plus 6 bolts per hour. Therefore, in one hour, both machines working together at their respective rates can produce x plus x plus 6 equals (2 x) +6 bolts. It follows that in 4 hours, both machines working together at their respective rates can produce 4 ((2x) +6) equals (8 x) +24 bolts.