It is a pair of trousers. or It is a trousers.誰是正確?

(pair of ) trousers, shorts, slack.
For attributive adjective naming a quality is used with the noun as in "of trousers"
(a) Some nouns have the same form in plural as in singular:-
eg:-deer
(b)Some nouns have no plural forms although they are not material or abstract nouns:-
eg:-luggage; often use "a piece" and "some pieces" to count them.
(c)Some nouns look singular but have a plural meaning:-
eg:-the young
(d) Some nouns look plural but are singular:-
eg:-news
(e)Some nouns are always plural:-
eg:-clothes,
pants,pyjamas,
shorts, trousers
pair of trousers
"some" pair of trousers
It is a pair of trousers.

It is a pair of trousers.

🔷trousers：[名詞]〔複數使用〕褲子（◆在美國：形式以外，使用 pants。
♥ <本題> 例1. It is a (new) pair of trousers  { 這是一條(新)褲子 }
例2. These trousers are dirty. { 這條(這些)褲子很髒。}
（◆1條或2條以上皆可使用；欲明示2條以上的事情之場合，使用 these pairs of trousers）
例3. be in short trousers { (我)還是孩子 }

根據Yahoo奇摩字典
https://tw.dictionary.search.yahoo.com/search?p=trousers

a pair of trousers 一條褲子

It is a pair of trousers.
因為牛仔褲係計雙數嘅

A pair of trousers is the correct form.  Trousers always in pairs.  褲子中文不分單、復數，英語褲子永遠是復數。

It is a pair of trousers.