I'm not sure how good I'll be at this, but I'll give it a try. I hope my response is of some help.
First, if you can, review the Flaw Reasoning chapter in the Power Score LR book. That chapter discusses the 17 different types of flaw questions and will help you learn how to identify right from wrong answers.
Second, break down the stimulus into premise(s) and conclusion. If you do this, you will see the "gap" between the first premise and second premise and the conclusion. The "gap" being that, "people are careless and do not date their wills...". This is a "gap" because the author does not connect the first premise to the second or the conclusion. S/he first discusses how people change their wills according to changes in their personal lives or financial situations then throws in, "people are careless and do not date their wills...".
Third, the flaw in this stimulus is that the author treats the "gap" as the only possible problem that can occur, when the reality is that several problems (i.e. the will does not address all possessions of the owner) can occur.
Fourth, since the author assumes that the "gap" is the only problem that can occur, s/he addresses that problem with only one solution which might not solve the problem (another solution might solve the problem but since the author does not address any further solutions, s/he is treating the solution s/he provided as the only or complete solution). Again, this is incorrect because other solutions exist, yet the author fails to acknowledge this. So answer A is correct.