In a dilute aqueous solution of hydrochloric acid (HCl), the following species are present:

a) H3O+ and Cl-
b) H3O+ and OH-
c) Cl- and OH-
d) Unionized HCl

The correct answer is option 'A', which means that in a dilute aqueous solution of hydrochloric acid, the species present are H3O+ and Cl-.

Explanation:

Introduction to Hydrochloric Acid:
Hydrochloric acid (HCl) is a strong acid that is commonly used in laboratories and industries. When it is dissolved in water, it dissociates into ions, resulting in the formation of a dilute aqueous solution of hydrochloric acid.

Dissociation of Hydrochloric Acid:
When hydrochloric acid is dissolved in water, it undergoes a process called dissociation, where the acid molecules break apart into ions. The dissociation of HCl can be represented by the following equation:

HCl + H2O -> H3O+ + Cl-

In this equation, one molecule of hydrochloric acid (HCl) reacts with a water molecule (H2O) to form a hydronium ion (H3O+) and a chloride ion (Cl-). The hydronium ion is a positively charged ion formed when a water molecule accepts a proton from the hydrochloric acid, while the chloride ion is a negatively charged ion formed when the hydrochloric acid donates a proton to the water molecule.

Presence of Species in Dilute Aqueous Solution of Hydrochloric Acid:
In a dilute aqueous solution of hydrochloric acid, the dissociation of HCl results in the formation of hydronium ions (H3O+) and chloride ions (Cl-). These ions are responsible for the acidic properties of hydrochloric acid. The hydronium ions contribute to the acidity of the solution, while the chloride ions remain as spectator ions and do not directly participate in the acidity.

Conclusion:
In summary, a dilute aqueous solution of hydrochloric acid contains hydronium ions (H3O+) and chloride ions (Cl-). These ions are formed through the dissociation of hydrochloric acid in water.