$A = \{\phi, \{1,2\},4,\{3\}\}$
The set $A$ is a subset of $B$ if every element that is in $A$ is also in $B.$ This is denoted by $A \subseteq B.$
- $\phi \subseteq A$
- $\{\phi\} \subseteq A$
- $\{\{1,2\}\}\subseteq A$
- $\{4\} \subseteq A$
- $\{\{3\}\}\subseteq A$
- $A\subseteq A$
Each object in a set is called an element of the set. The mathematical notation for "is an element of" is $\in.$ For example, to denote that $5$ is an element of the set $A$ of positive even integers, one writes $5 \in A$. To indicate that an element, $7,$ is not in the set $A,$ write $7 \notin A.$
- $\phi \in A $
- $\{1,2\}\in A$
- $4 \in A$
- $\{3\} \in A$
So, the correct answer is $(B).$