Final answer:
Option B, represented by the symbol ∅ (the empty set), does not belong to the set of real numbers because it contains no elements. All the other options are examples of real numbers.
Step-by-step explanation:
The student has asked which of the following does not belong to the set of real numbers: A. √3, B. ∅, C. -6.13, D. 1/3. The correct answer here is B. ∅. The symbol ∅ is used to denote the empty set, which does not contain any elements, let alone numbers, and therefore cannot belong to the set of real numbers. The real numbers include all rational numbers like 1/3 (option D), all irrational numbers like √3 (option A), and all whole numbers, integers, and decimal numbers whether positive or negative like -6.13 (option C).