All the 3 resistors below point A and B are in series. These 3 resistors can be replaced by an equivalent resistor of resistance R = 1+1+1 = 3 kΩ
Now, this R and 1 kΩ resistor are in parallel because they have common end points. And they can be replaced by an equivalent resistor of resistance
R' = 3x1/3+1 = 3/4
