Answer:
By changing the decimal to a percentage 
Step-by-step explanation:
Since the requirement is to calculate the percentage of students about their preference in eating vegetables, we need to convert the given values to percentage only. 
 Option A: By changing the addition sign to a multiplication sign. This is invalid because, there is no addition and multiplication involved in converting to percentage 
Option C: By changing the percentage to a decimal. The given data is not available in percentage so conversion is impossible. 
Option D: By changing the multiplication sign to a division sign Multiplication and division are not going to be performed here, since we need only percentage.