Reversed biased diode polarity in circuit
When the input voltage is greater than +1.5V the diode does not conduct so the output voltage is fixed. 

When the input voltage is less than or equal to +1.5V the resistors act as a voltage divider. Consider the diode a short and should be able to calculate the output voltage for any input voltage less than 1.5V.  

Hint: The change in output voltage will be 1/4 of the change in input voltage, but they are going to be equal at 1.5V in = 1.5V out.