function sum(num) {
if (num == undefined) {
num = 0;
}
return function (num2, num3) {
if (num3 === undefined) {
num3 = 0;
}
return num + num2 + num3;
}
}

The anwsers are correct, tested in console (A = 20 __ B & C = NaN __ C & D = 10)

The three options that can be invoked to get a return value of 10 are:

B. sum(10)()
This will set num to 10 and return the inner function that expects two arguments. When called without arguments, it will return 10.

C. sum(5, 5)()
This will set num to 5 and return the inner function that expects two arguments. When called with arguments num2 and num3 set to 5, it will return 10.

E. sum(5)(5)
This will set num to 5 and return the inner function that expects one argument. When called with an argument of 5, it will return 10.