(A) if they did not exploit its strategic location, their survival against the indomitable naval might of the British was always in peril —“was” is incorrect verb tense. Eliminate.
(B) if they did not exploit its strategic location, their survival against the indomitable naval might of the British could be liable to be in peril — “could” is incorrect usage here. Eliminate.
(C) If they did not exploit its strategic location, their survival against the indomitable naval might of the British would be always in peril — I’d have preferred would “always be” in peril. But the can tense is fixed here. Keep.
(D) if the were not to exploit its strategic location, their survival against the indomitable naval might of the British was always in peril — incorrect verb tense. “was”.
(E) their survival against the indomitable naval might of the British was always in peril, should they fail to exploit its strategic location — “was” is the incorrect verb tense here. It reduces the conditionality of the outcome ie survival against naval night of the British.
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