When carbon (X) is burnt in the presence of oxygen, it produces carbon dioxide which is neutral and acidic in nature as shown below. 2C+O2→2CO ‌‌‌‌‌‌‌‌‌‌‌‌‌(Neutral) C+O2→CO2 ‌‌‌‌‌‌‌‌‌‌‌‌‌(Acidic) Therefore, X is C.
