"Lesquelles sont tes préferées" or
"Lesquels sont tes préferés" -- Both mean which are those which you prefer? But more literally and probably clearer to understant the French connection would be: Which ones are your preferred ones?" Hence the subject is "Which ones = lesquelles/lesquels".

"Tes" needs to be in plural because it needs to match the plural form of "preferred ones = préferées/préferés".

In the first sentence you are talking about feminine things (e.g., chaussures) and in the latter about masculine things (e.g., chemins). But in either case you are talking about things in plural. This is indicated by "lesquelles/lesquels" (the plural form), "sont" (third person plural), tes (also plural) and "préferées/préferés" again plural.

So, to make it all hang together, it all needs to indicate the plural form.

Hope that helps, -- Chris (not a native speaker)

thanks much