A-potassium
B-lead
C-iron
D-magnesium
E-zinc

The unknown substance : Lead

Further explanation

Given

Density of element

mass of unknown substance = 40 g

volume = 3.5 cm³

Required

The density of unknown substance

Solution

Density is the ratio of mass per unit volume  

The unit of density can be expressed in g/cm³ or kg/m³  

Density formula:  

Input the value to find the density of an unknown substance :

ρ = 40 g : 3.5 cm³

ρ = 11.43 g/cm³

If we look at the table, the substance which has a density of 11.43 is Lead

The activity series is shown in the attached picture. if the element is below the other, then you can displace the other. following the activity series as a guide, the reactions are as follows: nickel and aluminum chloride3 ni + 2 alcl₃  → 3 nicl₂ + 2 albarium and copper bromideba + cubr  →  no reaction because cu is less oxidizable than bapotassium nitrate and aluminum3 kno₃ + al  → al(no₃)₃ + 3 ksodium phosphate and zincnaso₄ + zn  → znso₄ + na

Maybe you should get some from your teacher and learn more so you can get your grades up and maybe study a bit more