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.The mark of the sixth student is

The correct answer is (a) 30.5.

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 mark of the sixth student is 189 + 30.5 = 219.5.
Since the mark of the sixth student cannot be a fraction, the mark of the sixth student is 30.5.

Option (b) is incorrect because the mark of the sixth student cannot be 31.5.
Option (c) is incorrect because the mark of the sixth student cannot be 31.0.
Option (d) is incorrect because the mark of the sixth student cannot be 30.0.