Write an algebraic expression to model the unknown number situation.

To model the situation described where the sum of five and a number is represented, we can define the components as follows:

1. Replace “a number” with x (this represents the unknown number).
2. Replace “sum” with + (indicating addition).
3. Replace “five” with 5 (the constant number).

Putting this together, the correct algebraic expression is:

5 + x.