You want to optimize the cost of your Cognitive Search Service in Azure. Review the parameter below and choose the most appropriate service for your five indexers that run for maximum of four hours: Run smaller scale production workload
Ready to share infrastructure with others
Require maximum search units as three
Require maximum partition size as 2GB
Require maximum replicas as three
 

Answer options:

A.Free tier
B.L1 tier
C.Basic tier
D.S1 tier

Correct Answer: C
Option A is incorrect because Free tier is not meant for production workloads. The indexer runtime should not exceed 3 minutes. It only supports a max of 3 indexers. 
Option B is incorrect because L1 tier is meant for storage optimized scenarios. They use desiccated machines and hence cost more. Since the objective here is to optimize the cost, L1 tier is not the correct choice.
Option C is correct because the Basic tier can be used to run smaller Production workloads that do not require dedicated machines and storage. They can support 5 or 15 indexers with a max partition size of 2GB and max 3 replicas. It supports one partition per service and offers a max of three search units.
Option D is incorrect because S1 tier is used when the processing resource requirements are high and that is achieved through the dedicated hardware. They offer a max of 36 search units. Since the objective here is to optimize the cost, S1 tier is not the correct choice.
Reference:
To learn more about service limits in Azure Cognitive Search, use the link given below: 

https://docs.microsoft.com/en-us/azure/search/search-limits-quotas-capacity