Experiments designed to further our understanding of

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