The passage states that though second-hand shopping can help reduce pollution due to clothing ending up in landfills, such purchases are actually effective in saving the planet only in the case of high-quality second hand clothes, as low-quality older clothes cause microfibre pollution.

If second-hand clothes only sold high-quality clothes, then the central idea of the passage, that not all second-hand clothing purchases are effective is saving the planet, would be undermined. Option C is the right choice.

Primark and Boohoo are, according to the passage, 'fast fashion' brands. That is, they sell inexpensive, low-quality trendy clothing. If option A were true, then it would support (not undermine) the central idea that people should not simply purchase second-hand clothes without considering whether what they are doing is effective.

Even if options B or D were true, these options do not directly undermine the question of effectiveness of second-hand purchases in saving the planet. So, these options are ruled out.