(i) The driving force required to overcome the resistance is 300 N. The power \(P\) is given by the formula \(P = Fv\), where \(F\) is the force and \(v\) is the velocity.

Substituting the given values, \(P = 300 \times 40 = 12000 \text{ W} = 12 \text{ kW}\).

(ii) The power at 90% of the value found in part (i) is \(0.9 \times 12000 = 10800 \text{ W}\).

Using the formula \(P = Fv\), we find the driving force \(F\) when the speed is 25 m s^-1: \(F = \frac{10800}{25} = 432 \text{ N}\).

Applying Newton's second law, \(F - 300 = 1000a\), where 300 N is the resistance.

Thus, \(432 - 300 = 1000a\).

Solving for \(a\), we get \(a = \frac{132}{1000} = 0.132 \text{ ms}^{-2}\).