Whereas high speeds generally cannot be achieved by ramjets without their initially being assisted by a rocket, a scramjet, or supersonic combustion ramjet, can attain high speeds by reducing airflow compression at the entrance of the engine and letting air pass through at supersonic speeds.

(A) Whereas high speeds generally cannot be achieved by ramjets without their initially being assisted by
"without their initially being assisted by" is wordy; also comparison needs to be drawn between ramjets and scramjet
(B) Whereas a ramjet generally cannot achieve high speeds without the initial assistance of
Correct comparison drawn. Without initial assistance is concise and conveys the meaning
(C) With ramjets, high speeds generally are not able to be achieved without initially being assisted by
Contrast is not evident here. Also the option is way too wordy
(D) Unlike the ramjets, generally unable to achieve high speeds without the initial assistance of
By adding the important information within commas, it distorts meaning of the sentence
(E) Although a ramjet generally does not have the capability of achieving high speeds without the initial assistance from
Again wordy option; also "assistance from" is incorrect