Question

set a is a proper subset of set b, expressed as means that set a is a subset of set b and
ac b
a⊆ b
a⊄ b
a⊈ b

Explanation:

The symbol for "A is a proper subset of B" is $A\subset B$. A proper - subset means all elements of A are in B, but B has at least one element not in A. The symbol $A\subseteq B$ means A is a subset (could be equal), $A
subseteq B$ and $A
subset B$ mean A is not a subset and not a proper - subset respectively.

Answer:

$A\subset B$