¡Hola Shirley! It's a pleasure to meet you.
"Un pantalón" it's not wrong at all. This word is actually pretty interesting, you can choose to say either, "un pantalón" or "unos pantalones" to refer to a singular specific pair of pants. Even if you're not refering to more than 1 pair of pants, you can still say "unos pantalones".
Now, "este es la ultimo moda de los pantalones" is wrong because the word "moda" (fashion, trend) is feminine and the adjectives in the sentence need to be feminine as well. Having said that, the correct way to express it would be: "estA es la ultimA moda de los pantalones".
Hope this clarifies your doubts. If you have any more questions, please don't hesitate to contact me.
Hope you have an awesome evening Shirley!