What is 2^{3,b,x}?

a) Incorrect expression because an exponent should be a number, not a set;

b) Incorrect expression because the set-exponent has mixed numbers and symbols;

c) {, {3, b}, {3}, {b}, {b, x}, {x}, {3, x}, {3, b, x}}

d) {, {3}, {b}, {x}, {3, b}, {b, x}, {3, b, x}}

e) None of the above.