In Mirabilis jalapa when homozygous red flowered and white flowered plants are crossed, all F${}_1$ plants have pink coloured flowers. In F${}_2$ produced by selfing of F${}_1$ plants, red, pink, white flowered plants would appear respectively in the ratio of

The correct option is D 1:2:1
In the plant Mirabilis jalapa, commonly called as four o'clock plant, the inheritance of flower colour is an example for incomplete dominance. The plant produces two types of flowers red coloured and white coloured. This condition is an example for a pair of contrasting characters. When a plant which is homozygous for red flowers (AA) is crossed with a plant which is homozygous for white flowers (aa), the plants of the F${}_1$ generation produce pink flowers which is a blend of red and white condition. This result clearly indicates that neither red flowered condition nor white flowered condition is dominant. However, when two hybrid plants with pink flowers (Aa) are crossed, the F${}_2$ generation plants show red flowered, pink flowered and white flowered condition in the ratio 1:2:1. This ratio is very much in accordance with the law of segregation.
This example very clearly indicates
1. The phenomenon of incomplete dominance
2. That the genes responsible for red and white flowers do not actually mix, since both the pure characters reappear in the F${}_2$ generation
3. That there is no specific gene responsible for producing pink flowers
4. That the homozygous white flowered plants have genes aa which is unable to produce the colouring pigment
5. That the heterozygous pink flowered plants have genes Aa and hence can produce only half the amount of colouring pigment that is normally produced in a red flowered plant (AA).