Question

Haploids are preferred over diploids for mutation studies because :

Answer 1

Haploids possess only one pair of allele and diploid possess two pair of alleles.
The haploid allele will code for the particular gene and will always be expressed in $F_1$ generation no matter they are dominant or not, so the mutations in these haploid alleles will also be expressed in any case. This enables better studies.

So, haploids are preferred over diploids for mutation studies because recessive mutation is expressed in F1 generation.