The correct option is Chara.

To understand why Chara is monoecious, let us break down the concepts of plant sexuality:
1. Monoecious Plants: These are plants in which both male and female reproductive structures are present on the same individual plant body. In lower plants like algae, this is also referred to as a homothallic or bisexual condition.
2. Dioecious Plants: These are plants where the male and female reproductive structures are borne on separate individual plants (unisexual condition).

Now, let us examine each of the options:
• Chara: It is a multicellular green alga that is monoecious. Both the female reproductive organ, known as the nucule (oogonium), and the male reproductive organ, known as the globule (antheridium), are developed on the same plant body. In Chara, the nucule is typically located directly above the globule at the nodes of the plant.
• Carica papaya: Papaya is a classic example of a dioecious flowering plant. An individual papaya tree produces either exclusively male flowers or exclusively female flowers.
• Marchantia polymorpha: This is a dioecious bryophyte (liverwort). The male reproductive structures (antheridia) are borne on specialized stalks called antheridiophores on male thalli, while the female reproductive structures (archegonia) are borne on archegoniophores on separate female thalli.

Therefore, Chara is the correct answer because it contains both male and female reproductive structures on the same plant.