1.Despite是介系词，引导一个介系词片语，来修饰后面的句子。不可加子句

Despite the cold weather, she wears little as usual.-->(介系词+受词=介系词片语)

Despite the fact that he was busy, he came to the station to meet us. 如果一定要连接两个子句时，必须加上the fact 来当受词。

2.前面有不定冠词a Mexican carpet，所以用单数的one代表同一件东西。

Mrs. Smith wants to buy many Mexican carpets, but she can't afford to buy them.

Mrs. Smith needs one Mexican carpet, but she can't afford to buy it.

1.Despite是介系词,介系词后面要加名词,你的题目句子"he had failed 50 times"这是一具完整句子(主词+动词),所以要用连接词
(B)既然(题意不符)
(C)除非(题意不符)

2.这是一个冠词使用的观念
已经明确指出用 a 所以要用one来代替前面所提过的 a Mexican carpet