The Chief of the Defence Research Agency dismissed all allegations that the…

Concept

A sentence that dismisses "allegations of failure" needs a completion that affirmatively supports the claim that the mission succeeded. The strongest such completion is one that both meets the core success criterion and carries no negative qualifier of its own; a completion that meets the criterion but also carries a negative qualifier (like a delay) is a weaker, less complete justification, and a completion that describes an outright breakdown supports the opposite claim.

Application

Apply that test to each of the four completions:

• Landing at the target site sixty seconds ahead of schedule: the core criterion (hitting the target) is met, and "ahead of schedule" carries no negative qualifier — if anything, it reads as an efficient, trouble-free outcome. This is a complete, unqualified reason to dismiss the failure allegation.

• Landing at the target site two minutes behind schedule: the target is still hit, but arriving late is itself a minor qualifier that raises a question about whether everything went exactly as planned, so it is a weaker completion than an early, trouble-free hit.

• The missile failing to take off from the launcher: this is an outright breakdown, which confirms the failure allegation rather than dismissing it.

Cross-check

Since one specific completion (the early, on-target hit) already gives a complete and unqualified reason for the dismissal, the sentence does not need to fall back on "none of these"; among the remaining options, the late arrival is a comparatively weaker justification and the launch failure directly supports the opposite claim. The warhead landing at the target site sixty seconds ahead of schedule is therefore the correct completion.