The correct answer is B) change from smooth curves within the air mass to sharp bends at the front. Isobars, which are lines connecting points of equal atmospheric pressure, typically have smooth curves within a given air mass. However, when crossing a front, the isobars will bend sharply at the front itself. This is because fronts represent the boundary between two different air masses with distinct temperature and pressure characteristics, causing the isobars to change their pattern. The other options are incorrect because A) isobars do not pass through in straight lines at the front, C) isobars do not pass smoothly across the front without change, and D) isobars change from sharp bends within the air mass to smooth curves at the front, which is the opposite of the correct answer.