The correct answer is Option C: 27th.
Let's break down the problem step by step:
     1. Initial Situation:
            ○ There are 45 students in the class.
            ○ The boy is ranked 20th from the top.
     2. Change in Situation:
            ○ Two boys join the class, increasing the total number of students to 47.
            ○ The boy's rank drops by one, making his new rank 21st from the top.
     3. Finding the New Rank from the End:
            ○ To find his rank from the end, we subtract his rank from the total number of students.
            ○ New rank from the end = Total students - Rank from the top + 1
            ○ New rank from the end = 47 - 21 + 1 = 27
     Therefore, the boy's new rank from the end is 27th.