When a capillary tube is dipped in water vertically water rises to a height of 10 mm. The tube is now tilted and makes an angle 60⁰ with vertical. Now length of water column in tube is 

The volume of the water in the capillary tube increases while the vertical height of the water in the tube remains constant. 

vertical height h = $l\mathrm{cos\theta }$

$\Rightarrow l=\frac{\mathrm{h}}{\mathrm{cos\theta }}=\frac{10}{\cos 60}=\frac{10}{(1/2)}=20\mathrm{mm}$

Hence the correct answer is 20mm.