The correct answer is C. The application requires a solution that does not allow for any interruptions, which rules out the use of Spot Instances (Option B) as they can be terminated by AWS with a two-minute warning during periods of high demand. Option A, while providing a reserved instance, does not address the seasonal spikes specifically. Option D is not cost-effective as it reserves capacity for the peak usage all year round, which is unnecessary for short-term seasonal increases. Option C is the most cost-effective as it ensures the base load is covered by Reserved Instances, which are cheaper than On-Demand pricing, and allows the seasonal spikes to be handled by On-Demand Instances, which, despite being more expensive, do not require over-provisioning for the entire year and do not risk service interruption.