Final answer:
A deletion in chromosomes can unmask recessive alleles by eliminating the corresponding dominant alleles, revealing the recessive phenotype. This can be lethal but may also lead to the expression of the recessive traits in some cases. Therefore, the correct option is A.
Step-by-step explanation:
The chromosomal aberration that can result in the unmasking of recessive alleles resulting in their expression is deletion. A deletion involves the loss of a chromosome segment, which can eliminate the dominant allele counterpart to a recessive allele on the homologous chromosome, thus revealing or 'unmasking' the recessive trait. Deletions can cause a genetic imbalance which can be lethal to a diploid organism, but in cases where the deletion is survivable, it can lead to the expression of a previously masked recessive allele.