Mosfet switch implementation
The correct answer as I (and you) have calculated is 
$$
Z = \bar{A}\bar{B}\bar{C}
$$

The truth table is -

    A B C | Z
    - - - - -
    0 0 0 | 1
    0 0 1 | 0
    0 1 0 | 0
    0 1 1 | 0
    1 0 0 | 0
    1 0 1 | 0
    1 1 0 | 0
    1 1 1 | 0


Even if we think that the solution-maker has somehow forgot the 'manufacturing error' the answer comes out to be 
$$
Z = \bar{A}\bar{B}
$$
and the truth table is - 

A B C | Z
- - - - -
0 0 0 | 1
0 0 1 | 1
0 1 0 | 0
0 1 1 | 0
1 0 0 | 0
1 0 1 | 0
1 1 0 | 0
1 1 1 | 0