B and D are the correct answers.
I tested them in a JS compiler. Check the code below.

// Sample variables
var var1 = 10;
var var2 = 20;

// Option A
console.log(var1 + var2); // This adds numbers, not strings (result: 30)
console.log(String(var1 + var2)); // Converts to string after addition, result: "30"

// Option B
console.log(var1.toString() + var2.toString()); // Correct way, result: "1020"

// Option C
// Incorrect syntax — there is no 'string' object. It should be 'String'
try {
console.log(string.concat(var1 + var2)); // This will cause an error
} catch (e) {
console.log("Error: 'string' is not defined",e);
}

// Option D
console.log(String(var1).concat(var2)); // Correct way, result: "1020"

A,D tested

D instead of C