Which of the following plants is monoecious ?

The correct answer is Chara.

To understand why, let us break down the concepts of monoecious and dioecious plants:
1. Monoecious plants: These are plants that have both male and female reproductive organs on the same individual plant. In algae like Chara, the male reproductive structure (globule/antheridium) and the female reproductive structure (nucule/oogonium) are present on the same plant body, making it monoecious (homothallic).
2. Dioecious plants: These are plants where male and female reproductive organs are present on separate individual plants. Examples include:
- Carica papaya (Papaya): It has distinct male and female plants (dioecious).
- Marchantia polymorpha: This is a liverwort (bryophyte) that shows distinct male thallus (bearing antheridiophores) and female thallus (bearing archegoniophores), making it dioecious (heterothallic).

Therefore, among the given options, Chara is the monoecious plant.