How do I make conclusions from the autoradiograph of a Southern blot?
As @canadianer pointed out in the comments section flanking sequence is a sequence that is immediately near the target on either side. Since chimps and humans are closely related and the sequence different is minimal (according to this paper its approx 4% for the entire genome)  so using human primers is totally fine.  Now if take a look at the picture of the gel:
 The answer is 2. only A because only this offspring has a variant in large amount (that can be determined from the width of the band) that can be acquired from both parents and in this case chimp A actually did inherit the 136bp variant from both parents. 

For chimp 4 either parent can be the same (as for chimp A) from the two because it has the 136bp variant as well ( that can be inherited either chimp one or two), but has a different one that cannot come from chimps 1 and 2.

As you said chimp 5 can be ruled out instantly because it has a large band that cannot be found in either of the two parents.