突飛

彼の提案はあまりに ___ で、実現の可能性は低い。

The correct answer is 「突飛」(とっぴ), meaning "eccentric," "wild," or "far-fetched." When a proposal is described as 突飛, it suggests it is so unusual, unrealistic, or out of the blue that its practical implementation is highly improbable, perfectly fitting the consequence "実現の可能性は低い" (low possibility of realization). 「強硬」(きょうこう) means "firm" or "resolute"; a firm proposal doesn't inherently imply low feasibility. 「明白」(めいはく) means "clear" or "obvious"; an obvious proposal would generally be easier to understand and realize, contradicting the latter part of the sentence. 「緻密」(ちみつ) means "meticulous" or "elaborate"; a meticulous proposal would be well-thought-out and detailed, thus usually increasing its chances of realization, not decreasing them. Tip: 突飛 often describes ideas or actions that are startlingly original but lack practicality.

正解は「突飛（とっぴ）」です。「突飛」は、考えや行動が普通でなく、現実離れしていることを表し、「実現の可能性が低い」という文脈に合います。「強硬」は態度が強く譲らないこと、「明白」ははっきりしていること、「緻密」は細かく正確なことを意味し、いずれも文脈に合いません。