a) as a substitute of var
b) as a block statement to define new variables
c) to define variables that are scoped to a single expression
d) in a else if loop, as a substitute for var

Answer: d
Explanation: The let keyword can be used in four ways :
1. as a variable declaration like var;
2. in a for or for/in loop, as a substitute for var;
3. as a block statement, to define new variables and explicitly delimit their scope; and
4. to define variables that are scoped to a single expression.