To find density :-
PM=DRT
∴D=
RT
PM
Given :-
D
1
=2D
2
∣ M
1
=
2
1
M
2
∣ T
1
=T
2
D
2
D
1
=2 ∣
M
2
M
1
=
2
1
Now D=PM (∵R&T are constant)
D
2
D
1
=
P
2
M
2
P
1
M
1
2=
P
2
P
1
×
2
1
2×2=
P
2
P
1
∴
P
2
P
1
=4
Ans is C) 4