The correct answer is 'Spot Instances' because they allow customers to take advantage of unused EC2 computing capacity at steep discounts relative to On-Demand pricing. This option is suited for workloads with flexible start and stop times and can lead to significant cost savings. However, Spot Instances can be terminated by AWS with little notice if the demand for capacity rises, which is acceptable for the given scenario with intermittent and flexible bursts. 'Reserved Instances' would not be optimal due to the required long-term commitment. 'On-Demand Instances' are ideal for short-term and unpredictable workloads without any upfront payment or long-term commitment but they are more expensive than Spot Instances. 'Savings Plans' offer lower prices compared to On-Demand in exchange for a commitment to a consistent amount of usage (measured in $/hour) over a 1 or 3 year period, which doesn't fit the client's need for flexibility and lacks the deep discounts of Spot Instances for intermittent bursts.