The right answer is B.
Translation:
Jīntiān xièxie nǐ, yǒu kòng wǒ qǐng nǐ chīfàn.
Lǐ Huá：今天 谢谢 你， 有 空 我 请 你 吃饭。
Hey, thanks for your help today. If you have time I want to buy you dinner.
Méishìr, dàjiā dōushì tóngshì.
Tom：没事儿，大家 都是 同事。
Never mind, we’re colleagues.
A. 我不去 (Wǒbúqù): I’m not going.
B. 没事儿 (Méishìr): Never mind.
C. 不好意思 (Bù hǎoyìsi): Too obliged to come.
没事儿 (méishìr) is a very colloquial phrase to respond to others’ thanks or apologies. In the above dialogue, when Lǐ Huá thanks Tom for his help, the formal response would be 不用谢 (búyòngxiè), which means you’re welcome (or literally “don’t use thanks”). Tom chooses to use the more colloquial 没事儿 (méishìr) which actually acts to place the two as peers. In everyday situations, using 没事儿 (méishìr) is also completely appropriate.
Example:
Duìbuqǐ, wǒ chídào le.
Lucy：对不起，我 迟到 了。
I’m sorry, I’m late.
Méishìr. Wǒ yě gāng dào!
Lǐ Huá：没事儿。我 也 刚 到！
That’s all right. I just got here also.