Correct answer is [[5, 10, 15, 20], 5]
Key thing to note here is that any changes made to payload in for each loop are not available outside for each scope where as variable value updated in for each loop is visible out side for each loop too.

An object store is a facility for storing objects in or across Mule applications. Mule runtime engine (Mule) uses object stores to persist data for eventual retrieval. Internally, Mule uses object stores in various filters, routers, and other message processors that need to store states between messages.
Object stores are available in all deployment targets. If you deploy your application to CloudHub, you can also use Object Store V2.
Correct answer is Key/value pair in Object store
MuleSoft Documentation reference : https://docs.mulesoft.com/mule-runtime/4.3/mule-object-stores#use-cases