The correct option is C 0.6 M
Let volume of 1 M HCl solution is 4V Litres and 2 M HCl solution is V Litres. Then, molarity after mixing can be given as,
M′=M1×V1+M2×V2V′
where, M′ and V′ are molarity and volume of solution mixture,
V1 and V2 are 4V and V respectively.
So, M′=1×4V+2×V4V+V=1.2 M
Now, final volume is doubled after dilution, So final volume(Vf)=2V′=10V
∴ Mf=M′×V′Vf=0.6M