Is there a case when an atom of some element (e.g. carbon) does not have its expected number of protons (e.g. 6)? Page 12 of the Study Guide for Campbell Biology, 11th Edition has the following question:


  
  A carbon atom has 6 neutrons. How many protons are present in the nucleus of a carbon atom?
  
  A. 12 
  B. 8 
  C. 6 
  D. Can't tell; more information is required.
  


The answer given on page 431 is D. I think the answer should be C since carbon has 6 protons.

What type of additional information might be required?

I understand that elements are defined by their number of protons, but maybe there is some strange edge case like a moment during nuclear fusion? Is there a case when a carbon atom does not have 6 protons?
A carbon atom has 6 neutrons. How many protons are present in the nucleus of a carbon atom?
  


Obviously this wording would make any astute student say the answer is 6 since carbon by definition has 6 protons. You are correct, elements are defined by their number of protons. Key word being defined, changing the number of protons changes the element.

I believe the key here that your book is getting at is you only know that you have an atom from a carbon sample that has 6 neutrons but nothing else. I think the question is trying to evaluate if you are aware that proton numbers are independent of neutron numbers thus knowing there are 6 neutrons does not help you determine protons, especially given one of the options is:


  D. Can't tell; more information is required.


If this is what they wanted then it was a very poor choice of words to say a carbon atom in the question then ask for number of protons. They should have said:


  Given an atom (or nucleus) with 6 neutrons, how many protons are present in the nucleus of the atom?