Final answer:
The principal source of information about an applicant's identity, age, and marital status is the Application form. It collects all crucial identifying details and aids in due diligence by financial institutions.
Step-by-step explanation:
The principal source of information concerning an applicant's identity, age, and marital status is found in the B) Application form. When an individual applies for insurance, a loan, or a similar financial product, they must fill out an application form. This document is designed to collect all the important identifying information from the applicant. It usually includes spaces for the applicant's full legal name, date of birth, marital status, address, and sometimes other personal details such as Social Security Number or government-issued ID numbers. This information helps the financial institution to conduct due diligence and establish the identity of the person applying for their product or service.