If you want to stick to the if-else if construction, it's very easy as long as you order your conditionals a certain way (the way they're presented in the problem statement).

if (marks > 90)
    console.log("AA");
else if (marks > 80)
    console.log("AB");
else if (marks > 70)
    console.log("BB");
else if (marks > 60)
    console.log("BC");
else if (marks > 50)
    console.log("CC");
else if (marks > 40)
    console.log("CD");
else if (marks > 30)
    console.log("DD");
else
    console.log("FF");