Experiments
designed to further our understanding of lightning are not as applicable to "ball lightning" as they are to normal lightning, because it is so rare, unpredictable, and short-lived.
Thus the rduced stem :
EXPERIMENTS are not as applicable to Ball lighting as THEY are to Normal lighting, because IT is ...................
So THEY has a clear referrent = Experiments ,
But what does IT refer back to? ( Understanding the intent of the sentence gives the clue that IT should refer back to Ball Lighting ) ........... but the way IT has been placed creates ambiguity = Experiments or Ball lighting or Normal Lighting ( not clear )
Scanning of options :
A : wrong for IT ambiguity = Eliminated
B : IT (Sing) seems to wrongly modify / pounce on Experiments (plural) = Eliminated
C : Ball lighting cannot be studied by the same experiments as Normal lighting = Changes the meaning ( we don't know whether it can be studied or not = the case is of application ) = Eliminated
E : IT ambiguity apart from ..subject to experiments in the same way.......... = Eliminated
Left with D : because BL is so x, Y and Z.............., Experiments are not appliacble to IT (BL) as they are to NL = Perfect , clears the maze wrt ambiguity 7 has a logical flow.
Leading to D = my take