To solve this problem, we need to identify the metal used in the gene gun method for introducing alien DNA into host cells.$\newline$The gene gun method, also known as biolistic particle delivery, involves shooting microscopic particles coated with DNA into plant cells.$\newline$Let's analyze the options:$\newline \bullet$Option 1: Copper - Copper is not typically used in the gene gun method.$\newline \bullet$Option 2: Zinc - Zinc is not used in this method either.$\newline \bullet$Option 3: Tungsten or gold - These metals are commonly used as microparticles in the gene gun method.$\newline \bullet$Option 4: Silver - Silver is not used in the gene gun method.$\newline$The correct answer is Option 3: Tungsten or gold, as these metals are used to coat the DNA for delivery into host cells.$\newline$Therefore, the correct option is Option 3.