Which of the following statements best defines the monoecious?

The correct option is A flower with both androecium and gynoecium.

Understanding Monoecious and Bisexual Flowers:
In botanical terminology, the term monoecious (derived from Greek meaning "one house") is often used to describe plants that have both male and female reproductive organs on the same plant. When applied at the flower level, a flower that contains both the male reproductive whorl (androecium, consisting of stamens) and the female reproductive whorl (gynoecium, consisting of pistil/carpels) is referred to as a bisexual, hermaphrodite, or monoecious flower.
Let us analyze the given options to understand why this definition is correct:

1. A flower with gynoecium only: Such a flower is strictly female (unisexual) and is referred to as a pistillate flower.
2. A flower with androecium only: Such a flower is strictly male (unisexual) and is referred to as a staminate flower.
3. A flower with both androecium and gynoecium: This flower contains both male and female reproductive structures in a single flower, representing a bisexual state (often functionally referred to in simplistic terms as monoecious/bisexual).
4. A flower with dithecous: "Dithecous" refers to anthers that have two lobes (thecae). This is a structural characteristic of the male organ (stamens/androecium) and does not define the overall sexuality or monoecious nature of the flower.

Therefore, the statement that best defines monoecious (bisexual) in the context of the options is a flower containing both androecium and gynoecium.