(a) Pseudohypertropic muscular dystrophy is a recessive allele because if it were dominant then the heterozygous female would die before she was sexually mature, and thus could not pass on the trait
(b) Pseudohypertropic muscular dystrophy is a sex-linked trait because it only occurs in males.
(c) It is never seen in girls because for a girl to be homozygous recessive (and express the trait) XmXm, her father would have to be hemizygous recessive XmY) and her mother would have to be heterozygous or homozygous recessive (XmXM or XmXm). It is impossible for a homozygous recessive male or a homozygous recessive female to have children because they will die before being old enough to have children.