Step 1: Identify the tense and structure of the sentence.
The sentence uses the structure “would not have secured”, which clearly indicates a third conditional sentence referring to an unreal situation in the past.
Step 2: Recall the correct third conditional expression.
In third conditional sentences, the correct idiomatic expression used to indicate a negative past condition is “had it not been for”, which means “if it had not been for”.
Step 3: Eliminate incorrect options.
(1) if not for it was – grammatically incorrect word order and tense.
(2) unless it was for – incorrect tense and meaning for a past unreal condition.
(3) if it had been for – incorrect structure; does not express negation properly.
(4) had it not been for – grammatically correct, idiomatic, and perfectly fits the third conditional structure.
Step 4: Conclusion.
The sentence is correctly completed using “had it not been for”, which accurately conveys the intended meaning.