To find Sara's share of the rent, we need to divide the total rent \( r \) by the number of roommates (including Sara). Since Sara has three roommates, there are a total of 4 people sharing the rent (Sara and her three roommates).
The expression that represents Sara's share of the rent is:
\[ r \div 4 \]
So, the correct answer is:
B. \( r ÷ 4 \)