Answer:
192
1/192 = 0.5%
Explanation:
To calculate the total number of possibilities, we just have to multiply the number of possible items of each type, so
6 flowers * 8 vegetables * 4 herbs = 192 variations possible
There are then 192 ways to combine these plants into his garden.
Roses, tomatoes and basil is one of the 192 variations possible, so the probability for him to pick exactly this assortment is 1/192 or 0.5%