What is meant by the term isoelectronic series? A group of atoms, ions or molecules that have the same number of electrons but different nuclear charges (e.g. examples: Np⁴⁻, mg²⁺, Al³⁺, F⁻, n²⁻ all have 10 electrons).

Here are the solutions to your questions:

A)

(i) Give one example each of a:
(I) suspension: Muddy water (or sand in water, chalk in water)
(II) colloid: Milk (or fog, smoke, paint)

(ii) What is meant by the term isoelectronic series?
Your definition is correct. An isoelectronic series is a group of atoms, ions, or molecules that have the same number of electrons but different nuclear charges.
Your examples are also correct: $\text{Na}^+$, $\text{Mg}^{2+}$, $\text{Al}^{3+}$, $\text{F}^-$, and $\text{N}^{3-}$ all have 10 electrons.
✅ Correct

(iii) Define each of the following terms:
(I) heat of formation:
The standard enthalpy of formation ($\Delta H_f^\circ$) is defined as the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states under standard conditions (298 K and 1 atm pressure).