How are stuck at faults in a combinational circuits considered
When you want to check the consequences each individual stuck-at fault separately, a stuck at (0 or 1, to be considered separately) of the h input affects only that input. Likewise, a stuck-at of the i input affects only that input. A stuck-at of the g output affects both the i and h inputs, so these are 3 different cases (6 cases when 1 and 0 are considered separately). 

For the j output a stuck-at has the same effect as a stuck-at of the connected input of the final NAND, so there is no need to consider these separately.