Recessive alleles sometimes remain unexpressed because
another allele masks the recessive allele
recessive alleles rarely cross over
recessive alleles are very uncommon in the population
all of the above are correct

The answer is another allele masks the recessive allele.
Recessive alleles are only expressed when they are paired with another recessive allele. If there is a dominant copy of the allele present, that is the one that will be expressed instead. Recessive alleles are common in the population, probably one of the most well known recessive traits is blue eyes.