In some members of which of the following pairs of families, pollen grains retain their viability for months after release ?

The correct option is Rosaceae ; Leguminosae.

Pollen viability is the period during which pollen grains remain functional and capable of germinating to achieve fertilization. This viability varies greatly depending on the species and environmental factors such as temperature and humidity.
In some plant families, such as Rosaceae (rose family), Leguminosae (pea family), and Solanaceae (nightshade family), pollen grains can retain their viability for several months after their release.
In contrast, in members of the family Poaceae (grass family, including cereals like rice and wheat), pollen grains lose their viability very quickly, typically within 30 minutes of their release.

Looking at the options provided:
- Poaceae ; Rosaceae: Incorrect, as Poaceae pollen viability is very short.
- Poaceae ; Leguminosae: Incorrect, due to Poaceae.
- Poaceae ; Solanaceae: Incorrect, due to Poaceae.
- Rosaceae ; Leguminosae: Correct, as members of both of these families retain pollen viability for months.