Explanation:

To answer this question effectively, we must evaluate how to handle long-running, sequential operations (both database operations and web service callouts) in Apex without hitting governor limits, such as:

➜ The maximum CPU time (10,000 ms)
➜ The maximum number of callouts per transaction (100)
➜ The maximum number of DML statements (150)

Salesforce imposes strict governor limits to ensure multitenancy and performance. So to handle large, sequential operations, we need an architecture that breaks the work into manageable units, supports asynchronous execution, and allows chaining for sequential execution.

✅ Correct Answer: A. Use Queueable Apex to chain the jobs to run sequentially.

✅ Explanation:
Queueable Apex is ideal for running complex or long-running logic asynchronously. It allows you to:

➜ Perform large DML operations and web service callouts (which are allowed in Queueable jobs).
➜ Chain additional jobs using the System.enqueueJob() call from the execute() method.
➜ Maintain state between chained jobs using constructor parameters or custom logic.

This makes it perfect for sequential processing, where you need to perform one step, then move to the next only after completion (like billing → tax calculation → notification). You can organize each major step into a separate Queueable class and enqueue the next from within the current.

public class StepOne implements Queueable {
public void execute(QueueableContext context) {
// Do step one
System.enqueueJob(new StepTwo()); // Chain next step
}
}

🔗 Reference: Salesforce Docs – Queueable Apex

❌ B. Use Apex Scheduler to schedule each process.
The Apex Scheduler is designed for time-based or recurring operations, not for coordinating sequential steps within the same business process. Also, Scheduler jobs cannot be chained dynamically, so it’s not suitable for handling multi-step workflows that depend on the completion of prior steps.

❌ C. Use multiple @future methods for each process and callout.
@future methods are limited: no chaining, no guarantee of order, and only one web service callout per method.
@future also does not support returning results, making coordination harder.
Deprecated in favor of Queueable in many use cases.

❌ D. Use Limits class to stop entire process once governor limits are reached.
The Limits class allows you to check current resource usage, but does not prevent governor limits from being hit — it can only be used for fallback logic. Also, stopping the process mid-execution leaves the business process incomplete. This is more of a monitoring tool, not a control mechanism for architecture.

✅ Final Verdict:
A. Use Queueable Apex to chain the jobs to run sequentially is the best approach. It supports long-running, callout-heavy, and sequential processes while adhering to Salesforce governor limits.