a. A single replacement reaction takes place because zinc is more reactive and hydrogen
Further explanation
In voltaic series
Li-K-Ba-Ca-Na-Mg-Al-Mn- (H2O) -Zn-Cr-Fe-Cd-Co-Ni-Sn-Pb- (H) -Cu-Hg-Ag-Pt-Au
The more to the left, the metal is more reactive (easily release electrons) and the stronger reducing agent
The more to the right, the metal is less reactive (harder to release electrons) and the stronger oxidizing agent
So that the element located on the left can push the element on the right in the redox reaction
Reaction
Zn+2HCl ⇒ ZnCl₂ + H₂
If we look at this reaction, the elements that play a role are those that undergo a redox reaction
Oxidation : Zn⇒Zn²⁺ + 2e
Reduction : 2H⁺ + 2e ⇒H₂
So Zn (located on the left) can push H (the element on the right) in the redox reaction ⇒ Zn more reactive
A single replacement reaction is a chemical reaction in which one element replaces the other elements of a compound to produce new elements and compounds
A+BC⇒AC+B
So the above reaction involves the elements Zn and H which undergo a redox reaction and are included in a single replacement