Penetrance answers the question: of everyone who inherits the disease allele, how many actually show the disease? If the answer is less than $100\%$, the trait has incomplete (reduced) penetrance.
Practically, this produces a carrier who possesses the mutation yet looks completely healthy. In a family tree such a person sits between an affected ancestor and an affected descendant, so the condition seems to bypass a generation - affected individuals show up in alternate generations.
The other choices describe unrelated phenomena: a single affected sex points to sex-linked or sex-limited inheritance; a family with nobody affected simply does not carry the trait; and a pedigree where everyone in every generation is affected reflects fully penetrant dominant transmission. Note also that anticipation (progressively earlier/severe onset) is a separate idea and should not be confused here.
\[\boxed{\text{Incomplete penetrance} \rightarrow \text{trait skips a generation}}\]