Explanation
A. The argument does not assume that every volcanic ash cloud will reach an altitude of 5 kilometers; it only discusses what happens when clouds do exceed this altitude.
B. (Correct Response) For the argument to hold that lightning discharge data is sometimes the only reliable indicator of ash cloud altitude, it must be assumed that such data can be obtained in regions where weather radar, which also measures cloud altitude, is not available. If lightning discharges could not be detected in such regions, they could not serve as a reliable indicator of cloud altitude in the absence of weather radar.
C. The argument doesn't compare the accuracy of weather radar in measuring volcanic ash clouds to regular clouds. The availability of radar, not its accuracy, is discussed.
D. The argument does not claim that ash clouds exceeding 5 kilometers are likely to be beyond the reach of weather radar. It only notes that radar is not available in all parts of the world.
E. There is no assumption made about the frequency of lightning discharges in relation to the size of the volcanic ash clouds.
The underlying assumption necessary for the argument to work is option B. This allows lightning discharge data to serve as a reliable indicator when weather radar is not an option.