In this reaction which substance is oxidised and which substance is reduced ?

Solution:
This reaction is an acid-base reaction [Neutralization].
HCl is acid and CaCO₃ is a basic salt of Ca(OH)₂ (strong base) and H₂CO₃ (weak acid)
2 H⁺Cl^- + Ca²⁺CO₃^2- $\underset{}{\overset{}{\to }}$ Ca²⁺ 2Cl^- + H₂O + CO₂$\uparrow$
                                             (Salt)