Let me try to explain this -
(A)Not only do deep - sea drivers risk nitrogen narcosis ,often called "raptures of the deep",if they descend below 200 feet ,but
This option has the correct use of "Not only ...but also ". Each usage of "they" alludes to sea divers ( or is it drivers??) only and there is no pronoun ambiguity.
(B)Deep-sea drivers risk nitrogen narcosis,often called "raptures of the deep",if they descend below 200 feet,but
Here "but they also" is not idiomatic. There is no "Not only"
(C)Nitrogen narcosis,often called "raptures of the deep",is risked by deep-sea drivers if they descend below 200 feet,in addition
The option here wrongly suggests that Nitrogen narcosis is risked by deep-sea drivers. "Risked" is exposing something to danger. The meaning is clearly nonsensical.
(D)The descending of deep-sea drivers to below 200 feet causes them risking nitrogen narcosis,often called "raptures of the deep",and
Same as C + they also is not idiomatic.
(E)Not only does a deep-sea driver risk nitrogen narcosis,often called "raptures of the deep",if they descend below 200 feet,but
Same as D.
Hope this helps!