Answer:
Not a large enough sample
Step-by-step explanation:
Every 5th student going into the basketball game implies that 1/5 of the attendance would be surveyed, that make it a representative sample, so it is not a potential source of bias.
A biased sample implies that the sample doesn't represent the population. If you ask people if the school should spend more money on sports in a sport event, they are likely to say yes.
If you ask only students, not adults, you are avoiding people in purpose.
To get a representative sample, you should consider more than one sport.