(c) 90 m
Because,
Let us assume the Breadth of the rectangular field be x m.
Let us assume the length of the rectangular field is thrice its breadth be 3x m
Then,
Perimeter of the rectangular field = 2(l + b)
= 240 = 2(x + 3x)
= 240 = 2(4x)
= 240 = 8x
= x = 240/8
= x = 30
∴The length of the rectangular field is 3x = 3 × 30 = 90 m