< b="" />Explanation:

Monadelphous, diadelphous, polyadelphous, and syngenesious are terms used to describe the arrangement of stamens in a flower. Each term specifies a different pattern of stamen fusion.

Stamen fusion refers to the union of stamen filaments, which are the slender stalks that support the anthers. The anthers contain the pollen, which is the male reproductive structure in flowering plants.

Now let's analyze each option to identify the incorrect pair:

a) Monadelphous - Hibiscus:
Monadelphous means that the stamens are fused into one group or column by their filaments. This fusion can be either partial or complete. Hibiscus flowers have stamens fused into a tube-like structure, forming a column. Therefore, the pair Monadelphous - Hibiscus is correct.

b) Diadelphous - Cucurbita:
Diadelphous means that the stamens are fused into two groups or columns by their filaments. Cucurbita refers to the genus of plants that include pumpkins, melons, and gourds. In plants of the Cucurbita genus, the stamens are not fused into distinct groups or columns. Instead, they are free and separate from each other. Therefore, the pair Diadelphous - Cucurbita is incorrect.

c) Polyadelphous - Citrus:
Polyadelphous means that the stamens are fused into multiple groups or columns by their filaments. Citrus trees, such as oranges and lemons, have stamens fused into multiple groups. Therefore, the pair Polyadelphous - Citrus is correct.

d) Syngenesious - Helianthus:
Syngenesious means that the anthers of the stamens are fused together, forming a tube or ring around the pistil. Helianthus refers to the genus of plants that include sunflowers. In sunflowers, the anthers of the stamens are fused together, forming a tube around the pistil. Therefore, the pair Syngenesious - Helianthus is correct.

Conclusion:
The incorrect pair out of the options given is b) Diadelphous - Cucurbita. Cucurbita plants do not exhibit diadelphous stamen fusion, as the stamens are not fused into distinct groups or columns.