Which one can I choose?

Here is a glass of milk. Drink this quickly.
翻譯: 這裡有一杯奶,馬上飲掉這一杯奶吧!
1. 因為 a glass of milk 一杯,是數得到(countable)的一個單位,所以不是these眾數.
2. 又因為前面出現Here(這裡),所以不會是those(那些)。
3. 因此,this含的一個單位和這個,就完全符合上文下理的意思了。

2007-07-16 17:38:39 補充：
阿Tom仔,比晒解釋你架囉喎,重要問填邊個答案?查實喱條問題係好簡單,瞓身肯定/支持係this啦~你讀一讀上述啲三項解釋,係咪好啱無搵你笨吖。至於d點數唔知贈送比邊個,咪交付投票(voting)囉。

the answer is this (B)

Here is a glass of milk. Drink this quickly.

These and those are the plural form but the question said a glass that means we should use singular form and this is the singular form so you should use this

I think it is

Here is . Drink __ quickly.

B.this

Because (a) glass of milk