The correct answer is: A. plant is used for stand by purpose only.
A low utilization factor for a plant indicates that the plant is not being used to its full capacity. This could be because the plant is being used for stand by purposes only, or because it is under maintenance. If the plant is being used for stand by purposes only, this means that it is only being used when needed, such as during peak periods of demand. If the plant is under maintenance, this means that it is not being used at all while it is being repaired or upgraded.
Option B is incorrect because a plant that is under maintenance is not being used at all, so it cannot be said to be used for base load only. Option C is incorrect because a plant that is used for base load only is being used to its full capacity, so it cannot be said to have a low utilization factor. Option D is incorrect because a plant that is used for peak load as well as base load is being used to its full capacity, so it cannot be said to have a low utilization factor.