Let's solve the problem step by step.

Christina paints 420 square feet in 105 minutes. To find the unit rate, we divide:

$\frac{420 \text{ square feet}}{105 \text{ minutes}} = 4 \text{ square feet per minute}$

Now, looking at the answer choices, the correct option is:

✔ 4 square feet per minute

Would you like further explanation?

Here are five related questions to expand your understanding:

1. How do you determine a unit rate in a different context, like miles per hour?
2. What happens if Christina paints at a different speed? How would that change the unit rate?
3. How can you convert unit rates between different units (e.g., square feet per second)?
4. What if Christina painted for 200 minutes instead? How would you calculate her total coverage?
5. How do unit rates apply to real-world problems, like shopping or travel?

💡 Tip: Unit rates are useful for comparing different scenarios efficiently—always reduce rates to a "per 1 unit" form for clarity!