This problem is easiest to picture as a Venn diagram with two overlapping circles, one for Mathematics and one for English, sitting inside a rectangle that represents the whole group.
Adding up every region of the diagram gives the full group: only Mathematics (6) plus only English (8) plus both (4) plus neither (6), which is $6+8+4+6=24$.
Let's summarize:
Adding all four non-overlapping regions gives a total group size of 24 students, option (C).