Given that rate of work done is constant
If 1 machine can complete 1 order in 8 hours; it will complete x order in 1 hour
x=1/8

If 1 machine completes 1/8 order in 1 hour, then 4 machines will complete 1/8 * 4 in 1 hour
=> 4 machines will complete 1/2 in 1 hour

If it takes 1 hour for 4 machines to complete 1/2 of production order; it will take y hours for 4 machines to complete 1/1 of the order

$$\frac{1}{2}y=1$$
$$y=1\cdot\frac{2}{1}$$
$$y=2\ hours$$

$$Answer\ =\ A$$