Final answer:
The most suitable word to fill in the blank is 'Ongoing,' denoting that the peace negotiations were in progress at the time the border dispute occurred.
Step-by-step explanation:
The correct word to fill in the blank in the sentence 'Peace negotiations between the two countries were already ____ when the border dispute broke out.' is c) Ongoing. This is because the context implies that the peace negotiations were in progress but not yet settled or ended when they were interrupted by a new conflict. The options 'Fragile' and 'Tenuous' suggest a state of delicateness that could also be valid in many situations involving peace talks, but without a specific context pointing to the instability of the negotiations, 'Ongoing' is the most neutrally appropriate choice. 'Conclusive' does not fit, as it would imply the negotiations had been completed, contradicting the idea that they were disrupted by the border dispute.In summary, the main answer is Ongoing, which fits the general understanding of peace negotiation processes that are typically protracted, as evidenced by historical events such as the Korean War armistice discussions and the intricate Israeli-Palestinian peace talks. Both of these historical situations experienced extended negotiation periods marked by setbacks, underscoring the complexity of achieving peace agreements between conflicting parties.