Final answer:
The lender should insert the phrase 'Jointly and severally' after the names of the co-borrowers.
Step-by-step explanation:
The lender would be best advised to increase the security on the note by inserting the phrase Option D) Jointly and severally after the names of the co-borrowers.
This phrase means that each co-borrower is individually responsible for repaying the entire loan amount. If one co-borrower is unable to repay their portion, the lender can seek repayment from the other co-borrowers.
For example, if two siblings co-borrow a loan and one sibling defaults on their payment, the lender can pursue the other sibling for the full loan amount.