The correct answer is C. Majority of partners.
A new partner can be admitted into a partnership with the consent of a majority of the existing partners. This is because, under the law of partnership, a partnership is a voluntary association of two or more persons to carry on a business for profit. The partners have the right to admit new partners into the partnership, and the consent of a majority of the existing partners is sufficient to do so.
Option A is incorrect because it requires the consent of all existing partners. This is not always possible, as it may be difficult to get all of the partners to agree on a new partner.
Option B is incorrect because it allows any of the partners in the firm to admit a new partner. This could lead to a situation where a new partner is admitted without the consent of the other partners, which could disrupt the partnership.
Option D is incorrect because it requires the consent of two-thirds of the partners. This is a higher threshold than is required under the law, and it could make it difficult to admit a new partner.
In conclusion, the correct answer is C. Majority of partners.