What is the force in units of newtons of a 40 pound object? a ) 176 n b ) 9.1 n c ) 4.0 n d ) 400 n

There's 2.2 pounds in a kg. 
1 kg = 2.2 kg
x kg = 40 pounds.

2.2 x = 40
x =  40 / 2.2 
x = 18.2 kg

Now you need to change 18.2 kg into Newtons.
F = m * g
g = 9.81
m = 18.2
F = 18.2 * 9.81
F = 178 N

You should pick A. Who knows how and when things were rounded.

F = 852 N

Explanation:

We apply Newton's second law to the trailer :

F = m*a   Formula (1)

F : net force exerted by the truck on the trailer Newtons (N)

m : mass of the trailer in kilograms (kg)

a : acceleration of the trailer in meters over second square (m/s²)

Data

a=1.20 m/s² : acceleration of the trailer

m=710 kg : mass of the trailer

We replace data in the   Formula (1)  to calculate the net force exerted by the truck on the trailer

F = (710 kg)*(1.20 m/s²)

F = 852 N