To solve this problem, we need to understand the function of copper ions in copper-releasing IUDs.$\newline$Copper IUDs are a type of intrauterine device used for contraception. The copper ions play a crucial role in their mechanism of action.$\newline \bullet$Copper ions are released into the uterine cavity, where they have a toxic effect on sperm.$\newline \bullet$This toxicity affects the motility and viability of sperm, thereby reducing their ability to fertilize an egg.$\newline \bullet$The presence of copper ions creates an environment that is hostile to sperm, suppressing their motility and fertilizing capacity.$\newline$Therefore, the correct function of copper ions in copper-releasing IUDs is:$\newline$Option 4: They suppress sperm motility and fertilizing capacity of sperms.