Step 1: Understand monoecious plants.
Monoecious plants have both male and female flowers on the same plant, unlike dioecious plants where male and female flowers are on separate plants.
Step 2: Review the options.
- Cucumber is a dioecious plant, so male and female flowers are found on different plants.
- Coconut is a monoecious plant, so both male and female flowers are found on the same plant.
- Brinjal and tomato are also dioecious plants, with male and female flowers found on separate plants.
Step 3: Conclude.
Coconut is a monoecious plant, making option (B) the correct answer.
Final Answer:
\[
\boxed{\text{Coconut}}
\]