Final answer:
Rukmani spends money on sweets and treats as an extravagance besides the toy cart when they have a good amount of money.
Step-by-step explanation:
The "extravagance" that Rukmani spends money on, besides the toy cart, in the market when they have accumulated a good amount of money is c) Sweets and treats. Rukmani and her family, despite the varied and tempting merchandise around, opt to spend their hard-earned money prudently.
In the narrative, the moment of indulgence in the market reflects Rukmani's character and the choices she makes for her family amid their modest means. It is an example of how even in challenging times, little luxuries can bring joy and a sense of celebration to the family.