Solution :
We know that the time period of oscillation of a spring mass system is given by :
[tex]$T = 2 \pi \sqrt{\frac{m}{k}}$[/tex] , where m is mass and k is the spring constant
∴ [tex]$T_A = 2 \pi \sqrt{\frac{m}{k}}$[/tex] .........(i)
[tex]$T_B = 2 \pi \sqrt{\frac{2m}{k}}$[/tex] ..........(ii)
[tex]$T_C = 2 \pi \sqrt{\frac{3m}{6k}} = 2 \pi \sqrt{\frac{m}{2k}}$[/tex] ..........(iii)
[tex]$T_D = 2 \pi \sqrt{\frac{m}{4k}}$[/tex] ...............(iv)
Comparing the equations (i), (ii), (iii) and (iv)
We get
[tex]$T_B > T_A > T_C > T_D$[/tex]
So in increasing order of time period, we get
[tex]$T_D < T_C < T_A < T_B$[/tex]