A triangle of sides a, b, c must follow the rules:

Let's check the given options:
a. {11, 21, 33}:

b. {14,27,43}:

c. {11,13,23}:

d. {5,8,15}:

Answer: the set of numbers that could represent the three sides of a triangle is the third-one { 11, 13, 23 }