When the mother trees of rubber are evolved by cross-pollination (hand pollination) between two primary clones and are then multiplied vegetatively, they are called ___:

Correct option is B

Secondary clones are the result of cross-pollination between two primary clones of rubber trees.
· These secondary clones are then multiplied vegetatively to maintain the desired traits of the parent trees.
Explanation:
· (a) True clones – True clones are plants that are exact genetic copies of the parent plant, typically produced through vegetative propagation.
· (b) Secondary clones – Correct answer. Secondary clones are derived from cross-pollination between primary clones, followed by vegetative multiplication.
· (c) Tertiary clones – Tertiary clones refer to further generations of clones, but this term is not typically used in the context of rubber tree breeding.
· (d) Tributary clones – Tributary clones is not a standard term in rubber tree breeding.