2

Pairs is the plural so answer 2 is correct.

2

I'm going with both. Using the strict definition of couple which is 2, it would either be two pair of pants or two pairs of pants, plural pants, unknown quantity. Upon thinking further, if you have two tickets you say you have a couple tickets. Not a couple ticket. So the key is which word is the plural. Pair is singular, pairs is plural. A fun brain teaser, thanks. If you have two couples do you have a couple couple or a couple couples?

Depends if you want to follow English grammar or make it up for yourself.

In conversation in England #2 would be correct but other English speaking countries make up their own rules so who would know!

And you’d call them trousers!

I believe number one would be the more correct but does it really make a difference? Or have you worries about language Nazis?