The marks obtained by six students in a test are 12, 25, 18, 18, 20, The number of students whose marks are more than the average marks is

There are 2 students whose marks are more than the average marks: 25 and 20.

(a) is incorrect because only 1 student has a mark more than the average mark.
(c) is incorrect because 3 students have marks less than the average mark.
(d) is incorrect because there are only 6 students, so there cannot be 4 students whose marks are more than the average marks.