Final answer:
In Salesforce Apex, a single transaction can make up to 100 HTTP callouts or API calls. This limit is enforced to prevent excessive use of external services and ensure the platform's stability.
The correct answer is C.
Step-by-step explanation:
In the context of Salesforce Apex development, the number of HTTP callouts or API calls that a single Apex transaction can make is up to 100 callouts. This means that within a single execution context, developers are limited to making a maximum of 100 web service callouts to external services.
It's important to plan your integration and batch operations accordingly to stay within this limit, to avoid hitting governor limits in Salesforce. Exceeding these could result in exceptions and failures in your Apex code.