Answer:
(a) greater
Explanation:
When comparing numbers, it is helpful to have them in the same form. You could express them both as fractions with the same denominator, as percentages, as decimals, or any other form you like.
walk to school: 18%
close to school: 11/50 = 22/100 = 22% . . . (greater than 18%)
The portion of students who live within 1/2 mile of school is greater than the portion who walk to school.
_____
Additional comment
The % symbol essentially means /100.
Then 18% = 18/100 = 9/50. The fraction 11/50 is greater than this.