Answer: c)previous function calls are still open when the function calls it-self and the arguments of these previous calls still occupy space on the call stack
Explanation: Recursion is the function which call itself on its own. It is considered as the memory intensive function because it requires large space for the memory storage as it keeps the record of the previously called functions.
The recursion function need the memory storage for the storing of the variables,functions, addresses ,ports etc which will require high space , therefore the recursion is considered to be memory intensive function.