If Meselson and Stahl's experiment is continued for four generations in bacteria, the ratio of 
¹⁵ N / ¹⁵ N : ¹⁵ N / ¹⁴ N : ¹⁴ N / ¹⁴ N containing DNA in the fourth generation would be

Option (d) is correct. 
Meselson and Stahl found that DNA of the first generation was hybrid or intermediate (¹⁵N and ¹⁴N). It settled in caesium chloride at a level higher than the fully labelled DNA of parent bacteria (¹⁵N and ¹⁵N). 
The second generation of bacteria after 40 minutes, contained two types of DNA , 50% light (N¹⁴ N¹⁴) and 50% intermediate (N¹⁵ N¹⁴).
The third generation of bacteria after 60 minutes contained two types of DNA ,25% intermediate (N¹⁵ N¹⁴) and 75% light (N¹⁴ N¹⁴) in 1 : 3 ratio.
It can be assumed that the fourth generation after 80 minutes would contain 12.5% N¹⁵ N¹⁴ and 87.5% N¹⁴ N¹⁴ DNA in 1 :7 ratio.
Thus, if Meselson and Stahl's experiment is continued for four generations in bacteria, the ratio of ¹⁵N / ¹⁵N : ¹⁵N and ¹⁴N : ¹⁴N and ¹⁴N containing DNA in the fourth generation would be 0 : 1 : 7.