(a) Three
(b) Four
(c) Five
(d) Six
Ans : (b) Four
Explanation :
Families of the given plants :
Indigofera : Fabaceae
Sesbania : Fabaceae
Salvia : Lamiaceae
Allium : Liliaceae
Aloe : Liliaceae
Mustard : Brassicaceae
Groundnut : Fabaceae
Radish : Brassicaceae
Gram : Fabaceae
Turnip : Brassicaceae
The family having different length of stamens in their flower is Brassicaceae and Lamiaceae.
We are provided with four plants belonging to the above two mentioned families. So, Salvia, Mustard, Radish and Gram will have unequal length of stamens.