How a New Partner may be admitted into a partnership firm?

With the consent of all old partners

The correct answer is option 3- With the consent of all old partners.

According to the Partnership Act 1932, a new partner can be admitted into the firm only with the consent of all the existing partners unless otherwise agreed upon. With the admission of a new partner, the partnership firm is reconstituted and a new agreement is entered into to carry on the business of the firm.