Write down all the subsets of the following sets
(i) {a} 

(ii) {a, b} 

(iii) {1, 2, 3} 

(iv) $\varphi$

We know that number of subsets of any set is $2^n$ where $n$ is the number of elements in that set.
Every non-empty set has two subsets $\varphi$ and the set itself.

(i) The subsets of $\{a\}$ are $\varphi$ and $\{a\}$.

(ii) The subsets of $\{a,b\}$ are $\varphi ,\{a\},\{b\},and\{a,b\}$.

(iii) The subsets of $\{1,2,3\}$ are $\varphi ,\{1\},\{2\},\{3\},\{1,2\},\{2,3\},\{1,3\},and\{1,2,3\}$

(iv) The only subset of $\varphi$ is $\varphi$.