this is second conditional statement because with the condition given,the result could happen though it seems unlikely to have happened.
So with the condition(if they did not hold it),the sentence following it should have "would".
In the choice b,the same condition is written in a better way(without it) followed by "would"
i hope my point is clear.
Affording strategic proximity to the strait of Gibraltar, Morroco was also of interest to the French throughout the first half of the twentieth century because they assumed that if they did not hold it, their grip on Algeria was always insecure
B. without it their grip on Algeria would never be secure
C. their grip on Algeria was not ever secure of they did not hold it.
D. without that, they could never be secure about their grip on Algeria
E. Never would their grip on Algeria be secure if they did not hold it