The correct answer is C. debentures.
A debenture is a long-term loan that is secured by the assets of the company. A partnership firm is not a legal entity, so it cannot issue debentures.
A bank loan is a loan that is made by a bank to a business or individual. A partnership firm can raise funds by taking out a bank loan.
A partner loan is a loan that is made by one partner to another partner in a partnership firm. A partnership firm can raise funds by taking out a partner loan.
Partner capital is the money that is contributed by the partners to a partnership firm. A partnership firm can raise funds by increasing the amount of partner capital.