The average marks of eleven students is 31.0 and that of the first six students is 31.5, while that of the last six students is 30.Then the marks of the sixth student are:

The correct answer is (c).

The total marks of the 11 students is 11 x 31 = 341.
The total marks of the first 6 students is 6 x 31.5 = 189.
The total marks of the last 6 students is 6 x 30 = 180.
The marks of the sixth student is 189 + 180 = 369.
The marks of the sixth student is 369 / 6 = 61.5.
Since the marks of the sixth student cannot be a fraction, the marks of the sixth student is 61.

Option (a) is incorrect because the marks of the sixth student is not 31.0.
Option (b) is incorrect because the marks of the sixth student is not 31.5.
Option (c) is correct because the marks of the sixth student is 61.
Option (d) is incorrect because the marks of the sixth student is not 30.0.