I would really like some lunch but i have so much work to do.

你好,答案肯定是A, 请你好好查一下答案.

B和D错在句子中有and.如果用了if的话,句子后面不需要and来连接.
正确的说法是： If you tell me (if you would say to me其实很生硬,生活中没人用,但是语法没有问题) what you want, I can get it for you.

C错在不知所云,You will tell me... and I can 完全不make sense.

关于A语气上的问题, 其实完全没有问题,很多美国人都这么说.如果一定要婉转一点,他们会说
(Let me know) what you want and i can get it for you.

希望可以帮到你,谢谢~有什么问题在告诉我吧