The correct answer is: E. No error
The sentence is grammatically correct. The word “unless” is used to introduce a condition, and the word “given” is used to indicate that something is being provided. The word “to us” is used to indicate that the details are being provided to the speaker. The word “by now” indicates that the details need to be provided by a certain time.
The sentence can be paraphrased as follows:
We cannot handle this complicated case today unless full details are provided to us by now.