During Scoping the customer indicated that they needed customization to salesforce CPQ Due to a...

The customer asks for customization because of a legacy system process.

Salesforce methodology says:

First validate whether the requirement is truly necessary in Salesforce or whether the legacy process should be transformed.

Therefore, the first step is:

✔ A — Ask follow-up questions to ensure the legacy process has business justificationYou must determine:

Why the process exists today

Whether it is needed in the new system

Whether CPQ or Billing already support the requirement natively

Whether it introduces unnecessary technical debt

Only after justification should you:

Consider customization

Estimate hours

Add developers or change orders

Thus, A is the correct first step.