Answer from cs61c-ck (Allen Lee 16583554) for Question 3
The Xs mean don't cares, ie. the output is determined regardless of the value of those inputs.

o = !a + a*!c

The full truth table should have 2^3 = 8 rows. For each X, we cover a "multiple-of-2" possible rows.  The first row in the above truth table has 2 X's, so that covers 4 possible rows.  The next two rows have 1 X each, each covering 2 possible rows.  This makes 8 rows covered by the don't cares.  Thus, the table is complete.