Axial placentation is a type of arrangement of ovules within the ovary of a flower. In this type of placentation, the ovules are attached to the central axis of the ovary. Let's discuss the examples given and understand why option 'A' is the correct answer.

Tomato:
- Tomato belongs to the Solanaceae family and has axial placentation.
- The ovules in the ovary of a tomato flower are attached to a central column or axis.
- The placenta is located in the center of the ovary and the ovules are arranged around it.

Dianthus:
- Dianthus, commonly known as carnation, also exhibits axial placentation.
- The ovules in the ovary of a dianthus flower are attached to a central axis.
- The ovary is positioned superiorly, and the ovules are arranged in a radial pattern around the central axis.

Pea:
- Pea belongs to the Fabaceae family and shows axial placentation.
- In a pea flower, the ovules are attached to the central axis of the ovary.
- The ovary is positioned superiorly, and the ovules are arranged in a linear manner along the axis.

By analyzing the examples mentioned in option 'A', it is evident that all of them exhibit axial placentation. The ovules in these flowers are attached to a central axis or column within the ovary.

On the other hand, the examples given in options 'B', 'C', and 'D' do not exhibit axial placentation:
- Mustard, cucumber, and primrose show parietal placentation where the ovules are attached to the walls or partitions of the ovary.
- China rose, beans, and lupin exhibit marginal placentation where the ovules are attached to a single placenta that runs along the margin or edge of the ovary.
- China rose, petunia, and lemon also do not have axial placentation.

Hence, from the given options, the correct answer is option 'A' as all the examples mentioned (tomato, dianthus, and pea) demonstrate axial placentation.