The correct answer is: C. Mean 33; Median 35; Mode 35.
The mean is the average of the values in a set. To calculate the mean, add up all the values and then divide by the number of values. In this case, the mean is $33 = \frac{(20 \times 25) + (20 \times 30) + (40 \times 40)}{100}$.
The median is the middle value in a sorted set. To calculate the median, first order the values from least to greatest. In this case, the sorted values are $25, 25, 30, 30, 35, 35, 35, 40, 40, 40$. The median is the middle value, which is $35$.
The mode is the most frequent value in a set. To calculate the mode, count how many times each value appears in the set. In this case, the value $35$ appears 4 times, so the mode is $35$.
Therefore, the mean, median, and mode of marks obtained by the students are 33, 35, and 35, respectively.