Answer
Answer is $001$.
Work Step by Step
$$\text{Solution}$$
Here $A = 101, B = 100.$
Find $A - B = ?$ using 1's complement
$\rightarrow$ First, find 1's complement of $B = 100$
$\rightarrow $ Write out the numbers as if you were subtracting decimals.
$$-\begin{array}{c|c}
101 \\
100 \\
\hline
\end{array}$$
$\rightarrow $ Let's use the 1's Complement Method
$\rightarrow $ 1's complement of a number is obtained by subtracting all bits from $11$
$\rightarrow $ $\text{1's complement of 100 is}$
$$-\begin{array}{c|c}
111 \\
100 \\
\hline
011
\end{array}$$
$\rightarrow $ Now Add this 1's complement of B to A
Note $\rightarrow$ Addition Rules
$0 + 0 = 0$
$0 + 1 = 1$
$1 + 0 = 1$
$1 + 1 = 10$ ("$0$" in the column, carry $1$ to the next bit)
So,
$$+\begin{array}{c|c}
101 \\
011 \\
\hline
1000
\end{array}$$
$\rightarrow$ left most bit of the above result is called carry and add it to the rest part of the result $000$.
$$+\begin{array}{c|c}
000 \\
1 \\
\hline
001
\end{array}$$
so the Answer is $001$.
