Answer from cs61c-aj (Kevin Li 15855812) for Question 3
Xs means don't cares which means that no matter if its 0 or 1 the input or output will be the same for both 0 or 1 in place of Xs. 

o = !a + a.!c + !(a.c)

The table is complete because for all b and c where a is 0 is taken care by the first line. For a == 1, b can be anything and if c == 1 then o will be 0. For a == 1, b can be anything and if c == 0 then o will be 1. So for all cases no matter, a == 0 or 1, b == 0 or 1 and c == 0 or 1 is taken care of.