Answer:
(B). Gametangium.
Explanation:
The plant structure that produces gametes in lower, seedless plants, such as algae is known as gametangium.
Gemetangia that produce female gametes are known as archegonia, while gametangia that produce male gametes are known as antheridia.
These structures form gametes by the process of mitosis as lower plants reproduce asexually.
Thus, the correct answer is option (B).
