The error that this sentence makes is C. squinting modifier.
A squinting modifier is usually an adverb which makes a sentence somewhat unclear by changing the meaning of the words before or after it. So here, we aren't exactly sure whether the adverb <em>often </em>refers to driving cars often or the fact that driving cars can be expensive often.
The answer to your question is :
The cat is grey.
Hey Adam ,
I am sending you this email to apologize for not coming to your house to study together. I want you to know that I am really sorry but I really can’t come because my grandmother is in a terrible condition at the hospital and I want to visit her
Hope you understand,
Your friend Olivia
