The correct answer is: faith.
Faith is the belief in something without evidence. In the context of negotiations, faith in the sincerity of the other side is the belief that they are negotiating in good faith, with the intention of reaching an agreement that is beneficial to both parties.
Faith is important in negotiations because it allows the parties to trust each other and to make concessions without fear of being taken advantage of. Without faith, negotiations are likely to break down, as each party will be suspicious of the other’s motives.
The other options are not as relevant to the context of negotiations. Belief is a stronger form of faith, and certainty is the absence of doubt. While these qualities may be desirable in a negotiator, they are not essential. A negotiator can be successful without being certain of the other side’s sincerity, as long as they have faith that the other side is negotiating in good faith.
In conclusion, the correct answer is: faith. Faith is the belief in something without evidence, and it is important in negotiations because it allows the parties to trust each other and to make concessions without fear of being taken advantage of.