Why C? And why not the others?

Correct answer: C. EC2 Fleet (or Spot Fleet), which launches a fleet across multiple instance types, Availability Zones, and purchase options in one request

EC2 Fleet, and its predecessor Spot Fleet, is designed exactly for this scenario: a single request can define multiple instance types, span multiple Availability Zones, and blend On-Demand and Spot purchase options, and when the fleet includes Spot Instances it can automatically request replacement Spot capacity after an interruption, optionally using Capacity Rebalancing to proactively replace instances flagged as being at elevated risk of interruption before they're reclaimed. A single Launch Template tied to one instance type only describes the launch configuration for one kind of instance and has no fleet-level logic for spanning types, Availability Zones, or purchase options by itself. A Standard Reserved Instance is a billing commitment for one specific instance type in one specific Availability Zone (or Region, if regional) and neither launches instances nor mixes purchase options. A Dedicated Host reserves a single-tenant physical server for an account's own instance placement and has no mechanism for combining instance types, Availability Zones, or Spot and On-Demand purchasing within a single launch request.

Source: AWS EC2 documentation: EC2 Fleet and Spot Fleet — features and benefits