so as to表达目的:后面的谓语是缺少直接给出的主语,此时so as to 结构前出现主语和宾语时,谓语的施动者会有歧义,即:宾语还是主语发出了动作。这题中marry的施动者可以是king henry 也可以是queen caherine
so that 表达目的:that后要补出施动者,所以that 从句中的主谓是固定的,不会有歧义。
由此可见,so that前后的主语可以不同,表达的含义可以更灵活。
tip:当出现so that和so as to时,如果so as to没有歧义,且结构简洁明了,一般so that那个选项会有其他问题。不过不能一概而论
It looks like you copied choice D incorrectly - it should be just 'so that he could marry', making it the correct answer. The biggest problem with A is that it's not as immediately clear that HE wants to marry A.B. (it takes a couple of readings-through, for me at least, to figure that out for sure).
'So as to' isn't often the correct answer in GMAT-ville. It's somewhat wordy and awkward, although it isn't always wrong (I can't refer you to an OG problem off the top of my head where it's right, though).
This is a tough question. Honestly, if I saw this as an experimental question, my first reaction would be something along the lines of 'Choice A isn't wrong enough to be an actual wrong answer.'
Hopefully you understand it now. Remember to file this problem away in your mental list of things that the GMAT does and doesn't like.