Suppose a sexually reproducing population in which all individuals carry one deleteri-ous mutation ...

Suppose a sexually reproducing population in which all individuals carry one deleteri-ous mutation (one turn on Muller's ratchet, mutations can be the same or different). How can the ratchet be reset?
 
  a. An individual that is heterozygous for a deleterious mutation dies without leaving offspring.
  b. An individual emigrates from the population and removes a deleterious mutation from the gene pool.
  c. One individual that is heterozygous for a deleterious mutation leaves off-spring without the mutation.
  d. An individuals that is homozygous for a deleterious mutation mates with an individual that does not carry the same mutation.