a) 5 days
b) 6 days
c) 10 days
d) $$10\frac{1}{2}$$  days

Answer: c
Explanation:
$$\eqalign{ & \left( {{\text{B + C}}} \right){\text{‘s}}\,{\text{1}}\,{\text{day’s}}\,{\text{work}} \cr & = {\frac{1}{9} + \frac{1}{{12}}} = \frac{7}{{36}} \cr & {\text{Work}}\,{\text{done}}\,{\text{by}}\,{\text{B}}\,{\text{and}}\,{\text{C}}\,{\text{in}}\,{\text{3}}\,{\text{days}} \cr & = {\frac{7}{{36}} \times 3} = \frac{7}{{12}} \cr & {\text{Remaining}}\,{\text{work}} \cr & = {1 – \frac{7}{{12}}} = \frac{5}{{12}} \cr & \frac{1}{{24}}\,{\text{work}}\,{\text{is}}\,{\text{done}}\,{\text{by}}\,{\text{A}}\,{\text{in}}\,{\text{1}}\,{\text{day}} \cr & \frac{5}{{12}}\,{\text{work}}\,{\text{is}}\,{\text{done}}\,{\text{by}}\,{\text{A}}\,{\text{in}} \cr & {24 \times \frac{5}{{12}}} = 10\,{\text{days}} \cr} $$