A market found that$\frac{3}{7}$ of customers bought vegetables, $\frac{7}{13}$of customers bought fruit and $\frac{6}{11}$of customers bought canned food. Which purchase was made by a greater fraction of customers?

To compare all these values.

we need them to have the same denominator which means we have to find the GCF of the values given 

The GCF we got is 1001 

When we compare the values, 

we get 

Vegetables= $\frac{429}{1001}$

Fruits= $\frac{539}{1001}$
Canned foods= $\frac{546}{1001}$

So we can see that most of the customers bought canned food.

Hence, option C is the correct answer.