Read the sentence to find out whether there is any error in it. The error, if any, will be in one part of the sentence. The number of that part is the answer. If there is no error, the answer is (5). Ignore errors of punctuation, if any.

Sodium-ion batteries are efficient (a)/ and cheaper than Lithium-ion (b)/ batteries which have become (c)/ ubiquitous in the past three decades (d)/ No error (e).