The correct answer is: "d) 67.5%".
The turnover ratio is the number of occasions in which your stock of products has been completely sold during a given period of time. In other words, how fast it sells its inventory.
If in a given period, a company has managed to sell its inventory 4.5 times and each time it does, it gains a contribution margin of 15%, This means that the overall return of investment in that given period will be 4.5 times 15% or 67.5%.
In other words, the investor will get back the money he has invested in 1.48 years.