Universal Containers (UC) requires that all users specify a Contract is sent on each Opportunity...

The correct answer is C. Checkbox.

Universal Containers needs to track whether a contract was sent or not sent before an Opportunity is marked Closed Won. This is a simple Yes/No reporting requirement, so a Checkbox field is the best field type. Salesforce checkbox fields are represented as True/False values and can be used in reports and list view filters.

Picklist is unnecessary for a two-state value. Text and Text Area would create inconsistent reporting because users could type different values.