No, changing answer choice A to "The number of marriages has decreased since 1940" would not weaken the argument, since the question concerns the ratio of marriages to divorces.
For example, consider this scenario:

Let's say that divorce is the only cause of single parenthood. And let's say, every married couple has 1 child.
1940: 100 marriages, 10 divorces. So, 10% of the children have 1 parent
Today: 20 marriages, 5 divorces. So, 25% of the children have 1 parent

So, even though the number of marriages has decreased, the proportion of 1-parent children has still increased.

For this argument, the big assumption is that divorce is the only cause of single parenthood.
Answer choice E suggests another cause.

Cheers,
Brent

aditya8062 wrote:
the ratio concerned is of divorce to marriage and not vice a versa
my whole point was if number of marriages are reduced then also this ratio will increase and hence it might not be the case that number of divorce have increased as have been alleged by the argument

The argument is not suggesting anything about the number of divorces (or the number of marriages for that matter).
This argument is all about ratios and proportions.
The conclusion is that the proportion of children with 1 parent has increased. It is not suggesting that the number of children with 1 parent has increased.

Perhaps I'm not understanding your point.

Cheers,
Brent

ice_rush wrote:What is the source of this problem?


thanks!

The husband!!