​A Drosophila mutant (line A) with vestigial wings is isolated in a laboratory. The vestigial wing phenotype was observed to be recessive and mapped to gene 'X'. Three other laboratories also isolated vestigial mutants, called line B, C, and D. In order to test if the mutation in lines B-D also mapped to gene ‘X’, the following crosses were made and phenotype of F1 progeny observed.​

​Cross​	​F1 progeny (wing morphology)​
​A X B​	​Vestigial​
​A X C​	​Vestigial​
​A X D​	​Normal​
​B X C​	​Vestigial​
​B X D​	​Normal​
​C X D​	​Normal​

​Based on the above, identify the line(s) which is most likely NOT to have a mutation in gene ‘X’.​

Correct option is C

Explanation-

Given that:
The mutant with vestigial wings is mapped to gene X. Mutants B, C, and D are tested for allelism (whether they have mutations in the same gene, X). If two mutants are allelic (same gene) and recessive, the F1 progeny will be mutant (vestigial). If the mutations are in different genes, the F1 will be normal (complementation occurs).
Analyzing  the F1 results:
Crosses and F1 phenotypes:
    A × B → Vestigial   => B likely has mutation in gene X
    A × C → Vestigial   => C likely has mutation in gene X
    A × D → Normal    => D likely does NOT have a mutation in gene X
    B × C → Vestigial   => B and C likely have mutation in same gene (gene X)
    B × D → Normal     => Mutation in D is likely in a different gene than B (so not gene X)
    C × D → Normal     => Again, D differs from C, so likely not gene X
Conclusion:  Lines A, B, and C appear to have mutations in gene X.
Line D complements all others and shows normal wings in F1 => D does NOT have mutation in gene X.

Correct Answer:
Option c: Line D only