Step-by-step explanation:
Formation of compound -
A compound can be formed from its corresponding cation and anion ,
firstly the cation is written which is followed by the anion .
Then the valence of the cationic portion is written next to the anion as lowercase , 
and 
The valence of the anionic portion is written next to the cation as lowercase .
From the question , 
a. As³⁺ and Cl⁻ 
Here , 
cation = As³⁺
anion = Cl⁻ 
The compound formed by using the above information as, 
compound = AsCl₃
b. Sn²⁺ and P³⁻
cation = Sn²⁺
anion = P³⁻
The compound formed by using the above information as, 
compound = Sn₃P₂
c. Zn²⁺ and O²⁻
cation = Zn²⁺
anion = O²⁻
The compound formed by using the above information as, 
compound = ZnO
The valence of both cation and anion is same hence , gets cancelled out .
d. Mn⁴⁺ and S²⁻
cation = Mn⁴⁺
anion = S²⁻
The compound formed by using the above information as, 
compound = Mn₂S₄ = MnS₂