Final answer:
The unit rate of this proportional relationship is 2/3 mile per minute.
Step-by-step explanation:
To find the unit rate of this proportional relationship, we divide the distance by the time. Tomas rides his bike at a steady rate of 4 miles every 6 minutes. So, the unit rate is 4 miles divided by 6 minutes.
To simplify the unit rate, we can divide both 4 and 6 by 2. This gives us a unit rate of 2/3 mile per minute.
Therefore, the correct answer is B. 2/3 mile per minute.