Which one of the following is a gene mutation?

Nullisomy and Trisomy are mutations due to change in the number of chromosomes of a set or sets. Inversion is a chromosomal aberration caused due to tilting of a portion of chromosome to 180 degrees due to breakage and union of a portion of a chromosome which changes the sequence of genes on a chromosome but not the number and types. Transversion is a gene mutation in which a purine is replaced by a pyrimidine and vice-versa in a region within the gene.