An element X is a yellow solid at room temperature and shows catenation and allotropy. X forms oxides which are also formed during the thermal decomposition of ferrous sulphate crystals and are the major air pollutants. What is element X?

Sulphur is element X, a yellow solid at room temperature, showing catenation and allotropy. Sulphur forms oxides which are also formed during the thermal decomposition of ferrous sulphate crystals and are the major air pollutants. Carbon also is an element that shows catenation and allotropy. But carbon is not a yellow solid at room temperature.   
Hence, the correct answer is Sulphur.