Assertion. A variable not declared inside a function can still be used inside the function if it is declared at a higher scope level.

Reason. Python resolves a name using LEGB rule where it checks in Local (L), Enclosing (E), Global (G) and Built-in scopes (B), in the given order.

(a)

Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

Explanation
In Python, if a variable is not declared inside a function but is declared at a higher scope level (e.g., in the global scope or an enclosing scope), the function can still access and use that variable. Since Python follows the LEGB rule, variables declared at higher scope levels can be accessed within functions.