Answer: Option D
$$\eqalign{
& {\text{Let}}\,{\text{the}}\,{\text{cost}}\,{\text{of}}\,{\text{a}}\,{\text{chair}}\,{\text{and}}\,{\text{that}}\,{\text{of}}\,\,{\text{table}} \cr
& \,{\text{be}}\,Rs.\,x\,{\text{and}}\,Rs.\,y\,{\text{respectively}}. \cr
& {\text{Then}},\,10x = 4y\,\,\,or\,\,\,y = \frac{5}{2}x \cr
& \therefore 15x + 2y = 4000 \cr
& \Rightarrow 15x + 2 \times \frac{5}{2}x = 4000 \cr
& \Rightarrow 20x = 4000 \cr
& \therefore x = 200 \cr
& {\text{So}},\,y = {\frac{5}{2} \times 200} = 500 \cr
& {\text{Hence,}}\,{\text{the}}\,{\text{cost}}\,{\text{of}}\,{\text{12}}\,{\text{chairs}}\,{\text{and}}\,{\text{3}}\,{\text{tables}} \cr
& = 12x + 3y \cr
& = Rs.\,\left( {2400 + 1500} \right) \cr
& = Rs.\,3900 \cr} $$