For tracking whether contracts have been sent for Opportunities:

D. Checkbox. This field type allows users to clearly indicate (Yes/No) whether a contract has been sent. This binary nature of checkboxes makes it simple to filter and report on Opportunities based on whether a contract was sent.

Steps to configure this field:

Navigate to Setup → Object Manager → Opportunity.

Click Fields & Relationships → New → Checkbox.

Define the field with a label (e.g., Contract Sent).

Set the default value to false (unchecked).

Add the field to the relevant page layouts.

This setup facilitates accurate reporting and ensures that opportunities cannot be marked as "Closed Won" without indicating whether a contract was sent.

For guidance on creating fields, see Salesforce's documentation on Custom Field Creation.