Answer:
 (b)  56 ft
Step-by-step explanation:
The length is 4 more feet than the width of 12 feet, so is 12+4 = 16 feet.
The perimeter of a rectangle is twice the sum of length and width:
 P = 2(L +W)
 P = 2(16 ft +12 ft) = 2(28 ft)
 P = 56 ft
The perimeter of the sandbox is 56 feet.