A can do a piece of work in 4 hours, B and C can do it in 3 hours. A and C can do it in 2 hours. How long will B alone take to do it?

12 hours
A’s one hour’s work $latex = \frac{1}{4}&s=1$
(B + C)’s one hour’s work $latex = \frac{1}{3}&s=1$
(A + C)’s one hour’s work $latex = \frac{1}{2}&s=1$
∴ C’s one hour’s work $latex = \frac{1}{2}-\frac{1}{4} = \frac{2-1}{4} = \frac{1}{4}&s=1$
B’s one hour’s work $latex = \frac{1}{3}-\frac{1}{4} = \frac{4-3}{12} = \frac{1}{12}&s=1$
Hence B alone can do the work in 12 hours, so option [D] is correct answer.