04:00 Explanation: If the clock shows 3:45 and is 15 minutes slow, you add 15 minutes to the displayed time to find the actual time. So, 3:45 + 15 minutes = 4:00. Additionally, since it's mentioned that the actual time is a quarter past an hour, this confirms that the correct time is indeed 4:00 (since 4:15 would be a quarter past four, not a quarter past an hour from 3:45).