What is output?

public class StringLength $\{$

public static void lengthFun$($String str$)\{$

int length $=$ str$.$length$()$;

System.out.println$("$Length : $"+$ length$)$;

$\}$

public static void main$($String$[]$ args$)\{$

try $\{$

String str $=$ null;

lengthFun$($str$)$;

$\}$

catch $($Exception except$)\{$

System.out.println$($except$)$;

$\}$

$\}$

$\}$

a$.$

null

b$.$

$4$

c$.$

java.lang.NullPointerException

d$.$

java.lang.ArithmeticException