The question about plants with flowers having stamens of different lengths refers to a condition called "heterostyly" or "didynamous" and "tetradynamous" stamens in some cases. This is a characteristic feature in some flowering plants where the stamens (the male reproductive parts) are of unequal length.

Let's evaluate the plants listed :

1. Indigofera - Members of the Fabaceae family, like Indigofera, typically have uniform stamen lengths.
2. Sesbania - Also a member of the Fabaceae family, similar to Indigofera, likely has uniform stamen lengths.
3. Salvia - Belonging to the Lamiaceae family, Salvia exhibits didynamous stamens (four stamens with two different lengths).
4. Allium - A member of the Amaryllidaceae family, Allium species generally have stamens of equal length.
5. Aloe - A member of the Asphodelaceae family, Aloe species usually have stamens of equal length.
6. Mustard - Belonging to the Brassicaceae family, mustard plants exhibit tetradynamous stamens (six stamens with four of one length and two of another).
7. Groundnut - A member of the Fabaceae family, groundnut likely has uniform stamen lengths.
8. Radish - Part of the Brassicaceae family, radish plants also exhibit tetradynamous stamens.
9. Gram (Chickpea) - Belonging to the Fabaceae family, gram typically has uniform stamen lengths.
10. Turnip - As a member of the Brassicaceae family, turnip plants have tetradynamous stamens.

Based on this evaluation, the plants among the list that have stamens of different lengths in their flowers are Salvia, Mustard, Radish, and Turnip. Therefore, the correct answer is Option B : Four.