Which of the following plants is monoecious ?

Monoecious plants bear both male and female reproductive structures on the same individual, allowing a single organism to produce both types of gametes.

Among the listed options:

Marchantia polymorpha – a liverwort that is typically dioecious, having separate male and female gametophytes.

Cycas circinalis – a cycad whose reproductive organs are found on separate male and female plants, i.e., dioecious.

Carica papaya – some cultivars are monoecious, but the species is commonly considered dioecious with distinct male and female trees.

Chara – a genus of charophyte green algae. Individual Chara thalli develop both antheridia (male) and oogonia (female) on the same organism, meeting the definition of monoecious.

Therefore, the plant that is correctly identified as monoecious is Chara.