In plants, families are characterised on the basis of

The correct answer is both (a) and (b).

In plant taxonomy, plant families are characterized and classified based on a combination of different features to ensure accurate identification and classification. These features include:
1. Vegetative structures: These are the non-reproductive, vegetative parts of the plant, such as the leaves (their shape, arrangement, and venation), stems, and root systems. While vegetative characters are useful, they can sometimes be easily influenced by environmental factors.
2. Reproductive features: These are the floral characters, which include the structure of the flower, number and arrangement of sepals, petals, stamens, and carpels, inflorescence type, and placentation. Reproductive features are more stable and evolutionary conservative than vegetative features, making them highly reliable for taxonomic classification.

Therefore, plant families are characterized on the basis of both vegetative structures and reproductive features.