Suppose we have a rightmost derivation which proceeds as follows:
$\begin{array}{ccc}S &\rightarrow & Aabw \\ & \rightarrow &ABw \end{array}$
Which of the following is a possible handle for it?

$\begin{array}{ccc} A &\rightarrow & ab \end{array}$

$\begin{array}{ccc} A &\rightarrow & a \end{array}$

$\begin{array}{ccc} S &\rightarrow & A \end{array}$

$\begin{array}{ccc} B &\rightarrow & ab \end{array}$

Handle is part of the string in sentential form that will be reduced to non-terminal i.e left hand side of a production

In the above derivation, sentential form Aabw is reduced to ABw so has to be a production with B --> ab and that is the handle at this point of derivation.