Answer:
a random group of 40 people on the street
Explanation:
10 friends at a cookout would probably prefer grilled food.
The first 30 people leaving a hamburger stand probably do not care as much about vegetarian dishes as other people.
A random group of 5 people at a grocery store would possibly skew the data towards people who prefer fresh vegetables over other things.
The best choice is a random group of 40 people on the street.