It's "ne explétif". More info here: http://french.about.com/od/grammar/a/negation_form_2.htm
To understand the structure of the sentence in the exercise you need to be aware of the following two points:
à moins que is a conjunction meaning unless and requires the subjunctive
à moins que may use the ne expletive.
So what is the ne expletive
An expletive in grammar means a word or phrase used to fill out a sentence or line of verse without adding to the sense
ne is an expletive
The addition of a dummy ne is for euphony reasons only, i.e. for words to flow in a smoother way. This phenomenon belongs to formal French and it is found a lot in literature.
The ne explétif is used after certain verbs and conjunctions. It is used in situations where the main clause has a negative (either negative-bad or negative-negated) meaning, such as expressions of fear, warning, doubt, and negation.
The ne explétif is disappearing to some extent, and is more common in literary than in colloquial French, but it is still important to be able to recognize it so that when you do see or hear it, you understand that it does not make the subordinate clause negative (negated).
Elle a peur qu'il ne soit malade. - She's afraid that he is sick.
J'évite qu'il ne découvre la raison. - I'm avoiding his discovering the reason.
Nie-t-il qu'il n'ait vu ce film ?- Does he deny seeing this movie?
Il est parti avant que nous n'ayons décidé. - He left before we decided.
Luc en veut plus que Thierry n'en a. - Luc wants more than Thierry has.