8 idk. ......but.... 9 it leaves evidence and a mess
Answer:
Part A:
Charge is 
Configuration is 
Part B:
Charge is 
Configuration is 
Part C:
Charge is 
Configuration is 
Explanation:
Monatomic ions:
These ions consist of only one atom. If they have more than one atom then they are poly atomic ions.
Examples of Mono Atomic ions: 
Part A:
For P:
Phosphorous (P) has 15 electrons so it require 3 more electrons to stabilize itself.
Charge is 
Full ground-state electron configuration of the mono atomic ion:

Part B:
For Mg:
Magnesium (Mg) has 12 electrons so it requires 2 electrons to lose to achieve stable configuration.
Charge is 
Full ground-state electron configuration of the mono atomic ion:

Part C:
For Se:
Selenium (Se) has 34 electrons and requires two electrons to be stable.
Charge is 
Full ground-state electron configuration of the mono atomic ion:

The molecular formula : As₄S₆
<h3>Further explanation</h3>
Given
Rate of effusion of arsenic(III) sulfide = 0.28 times the rate of effusion of Ar atoms
Required
The molecular formula
Solution
Graham's law: the rate of effusion of a gas is inversely proportional to the square root of its molar masses or
the effusion rates of two gases = the square root of the inverse of their molar masses:

or

Input the value :
1 = Arsenic(III) sulfide
2 = Ar
MM Ar = 40 g/mol
0.28 = √(40/M₁)
M₁=40 : 0.28²
M₁=510 g/mol
The empirical formula of arsenic(III) sulfide = As₂S₃
(Empirical formula)n = molecular formula
( As₂S₃)n = 510 g/mol
(246.02 g/mol)n = 510 g/mol
n = 2
So the molecular formula : As₄S₆