The correct option is "many". The word "many" is used to refer to a large number of countable objects, such as "flowers".
The sentence becomes:
The petals of many flowers expand in the sunshine.
The correct option is "many".
Arun’s present age in years is 40% of Barun’s. In another few years, Arun’s age will be half of Barun’s. By what percentage will Barun’s age increase during this period?