Explanation:

- Option 1: The plaintiff sues as owner of the land.

- In a suit for possession, the plaintiff claims to be the owner and seeks possession as such, not as a mortgagor seeking redemption.

- Option 2: The plaintiff sues as mortgagor.

- A suit for redemption is filed by a mortgagor wanting to reclaim the property after repaying the mortgage, a different cause of action than suing as an owner for possession.

- Option 3: Both (A) and (B).

- Both reasons are correct. A suit for possession (as owner) and a suit for redemption (as mortgagor) have different legal bases, so one can't be simply converted into the other.

- Option 4: None of these.

- Incorrect because option 3 properly explains why conversion isn’t allowed.

Option 3 is correct.