The element common to all acids is A. Hydrogen B. Carbon C. Sulphur D. Oxygen

Hydrogen is the most common element of all the acids. There are some acids which don't have Carbon, Oxygen. But there is none without Hydrogen. Lets take a few examples to make it simple:
HCl - hydrochloric acid; HNO3 - nitric acid; H2SO4 - sulfuric acid
HBr - hydrobromic acid
HI - hydroiodic acid
HClO4 - perchloric acid
HClO3 - chloric acid
H2SO3 - sulfurous acid
HSO4 - - hydrogen sulfate ion
H3PO4 - phosphoric acid
HNO2 - nitrous acid
HF - hydrofluoric acid
HCO2H - methanoic acid
C6H5COOH - benzoic acid
CH3COOH - acetic acid
HCOOH - formic acid According to the Brønsted–Lowry acid-base theory and also the Arrhenius acid-base theory all acids contain Hydrogen.