Snakes are predators and the feed mainly on small mammals.
The poison is not an example of resistance because it is not used for protection or to resist in the absence of the ideal conditions, therefore A is incorrect.
The poison is also not a reptile and it isn't even an exclusivity of reptiles, therefore B is also incorrect.
A biotic factor is a living organism that shapes the environment, but the poison is not a living organism, but something that the organism can produce. Therefore, C is also wrong.
So, we can conclude that the poison is an adaptation, as it is an advantage acquired through natural selection that allows the the snakes to bring down and capture their pray. Therefore, the correct answer is