The correct option is D No sooner had I looked away than she fell down.
If the second event occurs immediately after the first, no sooner...than is used to express such situations. In such a structure, no sooner introduces the event that occurred first. No sooner is followed with the past tense. Option C is correct as no sooner is followed by 'had'. Options A is grammatically wrong. Option B is wrong as 'but' cannot be used with no sooner.