Which of the following is considered to be an acid a) hci b)koh c)naoh d)ca(oh)2

The correct answer is option A, that is, HCl.  

An acid refers to an ion or molecule possessing the tendency of donating a hydrogen ion or a proton, or alternatively, exhibiting a tendency of producing a covalent bond with a pair of electrons. The aqueous Arrhenius acids exhibit characteristic features that offer a practical illustration of an acid.  

The acids can turn blue litmus paper red, can form aqueous solutions with a sour taste, and reacts with bases and some kind of metals to produce salts. One of the general aqueous acids is HCl (hydrochloric acid) that is found in the stomach and stimulates digestive enzymes.  

practical process described pest part of scientific investigation.

because practical knowledge is powerful then therotical knowledge.