I haven't ---idea how that terrible accident could have happ

B。the least
Not the least 的意思是“毫不”、“一点也不”,也可写作 not in the least。但后者一般不用作定语。
i have not the least idea。我一点主意也没有。
又如：Are you cold?—Not（in）the least．
　　你冷吗?—一点也不冷。