David ranked 19th from the top and 25th from the bottom in his class. How many students are there in his class?

Given: 

David ranked 19th from the top and 25th from the bottom in his class.

Total students = Rank from top + Rank from bottom − 1

Total = 19 + 25 − 1 = 43

So, 43 students are there in his class.

Thus, correct option is (b).