Which dress do you like best,madam?sorry I can not decide now
a to buy which one b.by which one c which one to buy d which i should buy it
选项是C.
1.decide to do sth的意思是：“决定去做某事”
2.decide sth的意思是：“决定某事”
3.decide which one to buy的意思是：“决定买哪件”.【不定式to buy】做后置定语,修饰【which one】.