The given boolean expression is:
\[ a'b' + ab + a'b \]
Let's simplify it step-by-step:
- First, notice that the expression can be grouped as follows:
\[ a'b' + ab + a'b = a'(b' + b) + ab \]
- From the **complement law**: \( b' + b = 1 \), so the expression becomes:
\[ a' \cdot 1 + ab = a' + ab \]
- Now, we use the **absorption law**: \( a' + ab = a + b \).
Thus, the simplified expression is:
\[ a + b \]
Hence, the correct output is **(4) \( a + b \)**.