If $A=\{x:x\text{ is a letter in word BELOW}\}$, $B=\{x:x\text{ is a letter in word WOOL}\}$ and $C=A-B$, then the number of subsets of $C$ is

First, let's write sets $A$ and $B$ in roster form by listing their unique distinct element letters:$A=\{\text{B, E, L, O, W}\}$$B=\{\text{W, O, L}\}$ Next, compute the relative set difference $C=A-B$, which includes elements present in $A$ but completely absent from $B$:
$$C=\{\text{B, E, L, O, W}\}-\{\text{W, O, L}\}$$
$$C=\{\text{B, E}\}$$
The cardinality (number of elements) of set $C$ is $n(C)=2$.The total number of subsets for any given set containing $n$ elements is given by the formula $2^n$:
$$\text{Number of subsets of }C=2^{n(C)}=2^2=4$$