1.随便吃。
All-you-can-eat.
buffet
smorgasbord
Eat as much as you like.
Eat as much as you wish. *作为句子应为“你想吃多少就吃多少”。

2.多项选择。
multiple-choice
multiple-choice questions (多项选择题)

3.各就各位，预备，跑!
Ready, get set, go!
On your mark, get set, go!

4.从头到尾。
from A to Z. *是开始字母A到最后一个字母Z，即“从开始到最后”、“完全”的意思。
I don't know anything about driving. (我一点也不懂驾驶。)
I'll teach you from A to Z. (我会从头到尾教你的。)
entirely (完全地)
completely (完全地)