The circumstances described in the source were most directly facilitated by which development in Eurasia during the thirteenth century?

Bar Sauma, a monk from Mongol-ruled China, traveled overland to Persia and served an ilkhan as envoy to Constantinople, Paris, and England. That journey was facilitated by Mongol conquests and successor khanates, which linked much of Eurasia under related regimes and eased long-distance movement, so D is correct. Choice A is tempting because Indian Ocean trade mattered, but it was maritime and not his route. Choice B describes Mamluk power, which was regional. Choice C names Timurid rule, which expanded later than his late-thirteenth-century mission.