新幹線は飛行機___安いです。
Q. Which particle expresses 'cheaper THAN the plane'?
Translation
The Shinkansen is cheaper than the plane.
Explanation
より marks the standard of comparison: 飛行機より安い means 'cheaper than the plane.' ほど appears in other comparison patterns, including 飛行機ほど高くない ('not as expensive as the plane'), but that is a different sentence structure.