First, imagine that if there were 100 students in the science club in total, there would be 40 students who want to buy the T-shirt. That's what the number 40% tells us.
65 students in total* ( 40 students buying T-shirts/ 100 students in total)= 26 students buying T-shirts.
Note that students in total (in 65 students in total) and students in total (in 100 students in total) cancel out, and each student only buys one T-shirt.
Thus the final answer is D. 26
Hope this would help :))