Which of the following is a plausible mechanism for the arisal of the autohexaploid type?

a) union of reduced gametes from autooctaploid and diploid parents
b) union of unreduced gamets from autotriploid and autopentaploid parents
c) union of two unreduced gametes from two diploid parents
d) union of reduced gametes from autotetraploid and autooctaploid parents
e) None of the above