The total number of students is 20. Since 15 passed in English and 18 passed in Mathematics, the number of students who passed in at least one subject is:
But there are only 20 students, so some students must have passed in both subjects. Thus, inference I is correct.
To determine whether no student failed in both subjects, we check:
Since there is a possibility that all students passed at least one subject, inference II might seem correct, but it is not explicitly stated in the given statement. Thus, inference II does not necessarily follow.
Since only inference I follows, the correct answer is: