A student is writing an argumentative essay about adopting shelter animals instead of buying pets. Which detail would not be relevant to the argument?

1. Adopting shelter pets help save lives
2. Cats are more popular pets than dogs
3. Many dogs and cats need homes
4. Animals from pet stores are often unhealthy

2. Cats are more popular pets than dogs

While this detail may be interesting, it is not relevant to the argument of adopting shelter animals instead of buying pets. The focus of the essay is on the benefits and importance of adopting animals from shelters, not on comparing the popularity of cats and dogs as pets.

To determine which detail would not be relevant to the argument, we need to identify the main point or thesis of the argumentative essay. In this case, the main point is that adopting shelter animals is preferable to buying pets.

1. "Adopting shelter pets help save lives" - This detail is directly related to the argument because it supports the idea that adopting shelter animals is beneficial.

2. "Cats are more popular pets than dogs" - Although this detail doesn't directly address the argument, it might be relevant if the essay discusses why adopting shelter cats specifically is important. So, this detail could be relevant to the argument.

3. "Many dogs and cats need homes" - This detail is clearly relevant because it highlights the demand for homes for shelter animals, which supports the argument in favor of adopting them instead of buying.

4. "Animals from pet stores are often unhealthy" - This detail is also directly tied to the argument as it argues against buying pets from pet stores, reinforcing the idea that adopting from shelters is a better option.

Based on this analysis, it appears that all the details provided would be relevant to the argument. Therefore, none of the details would be considered as not relevant to the argument.

The detail that would not be relevant to the argument is "Cats are more popular pets than dogs."