Final answer:
Bilaterally symmetrical flowers are typically regular, meaning they have symmetry that allows for multiple planes of division that result in mirror-image halves.
Step-by-step explanation:
The question asks which characteristic is common to all bilaterally symmetrical flowers. Bilaterally symmetric flowers are those where a single plane can divide the flower into two mirror-image halves. This characteristic refers to the symmetry of the flower, not to the position of the ovary relative to the other parts of the flower. Therefore, we cannot assume that all bilaterally symmetrical flowers are hypogynous (having an ovary above the other flower parts), superior, inferior (having an ovary beneath other flower parts), or have any particular ovary position.
However, the symmetry term that does apply is regular or actinomorphic, as these flowers tend to have radial symmetry allowing for multiple planes of division that results in mirror-image halves. This is the typical feature of bilaterally symmetrical flowers.
Therefore, the correct answer is: b. regular.