Many people agree that ___ knowledge of English is a must in

当knowledge指一个人对某一具体事物或具体学科的知识时,可以与不定冠词a连用（不管有无定语）
如：He has an wide knowldge of history.
当knowledge泛泛地指知识的时候,它是个不可数名词,不能与不定冠词a连用
如：Knowledge is power.
a knowledge of 指的是比较广泛的某门学科的综合知识.
而the knowledge of指的是比较具体的某种办法的知识,比如The Knowledge of Life（生活小常识）,The Knowledge of Healing（疗愈的知识）.
举个例子：
a knowlede of medicine指的是整个医学知识,包括学习整个医学理论和实际给病人看病.
The Knowledge of Healing指的就是一些方法,比如咳嗽了怎么办,腰痛怎么办之类.
英语知识应该是一个综合的知识,它可以包括英语会话和英语社交常识等等,所以用a

Many people agree that _the__ knowledge of English is a must in _the___ international trade today.

a knowledge of sth 是个固定搭配，意为“某一方面的知识”，eg：a superficial knowledge of money 对金钱肤浅的认识；a good knowledge of geography 通晓地理知识；后面的international trade 指的是“国际贸易”是个专业名词，不需要加冠词来修饰的。所以答案是A
此句意为“许多人都赞同在国际贸易中掌握...

固定搭配，a knowledge of sth.