Explanation

Reserved Instances are the correct choice for this scenario because:

• Long-term commitment: Reserved Instances provide significant discounts (up to 75%) compared to On-Demand pricing when you commit to a 1-year or 3-year term
• Continuous availability: For applications that need to run continuously for 3+ years, Reserved Instances guarantee capacity and provide cost savings
• Predictable billing: Reserved Instances offer predictable pricing for long-term workloads

Why other options are incorrect:

• Spot Instances: These offer the lowest prices but can be interrupted with short notice, making them unsuitable for applications requiring continuous availability
• On-Demand Instances: These provide no discounts and are the most expensive option for long-term usage
• EC2 Fleet: This is a management feature that allows you to manage capacity across different instance types and purchasing options, not a specific discount pricing model

For workloads with predictable usage patterns and long-term requirements, Reserved Instances provide the best cost optimization while ensuring availability.