Spot the Error.

He said that he was suffering from fever since three days.

The given sentence is in the past tense and talks about an event that started in the past and continued up to another point in time [present/past]. The correct way to talk about such an event is to use the perfect continuous tense, which is formed using the "have/has/had been" + verb-ing form.

The sentence "He said that he was suffering from fever since three days" uses "was" [simple past tense]. However, since the sentence talks about an event that started in the past and continued to the present, it should use the present perfect continuous tense instead. The correct construction would be "has been suffering." 

Therefore, the correct sentence would be, "He said that he has been suffering from a fever since three days."

Therefore, Option B has an error.