Finding the output of a node with two diodes (kind of full-wave rectifier)
Steps to get your answers:

Look at the diagram. In the simple diode model (fixed 0.6V drop) the diodes act like pressure (voltage) operated valves: any voltage over 0.6V (difference between the two diode pins in forward direction) will be shorted (bleeded). When there is less voltage over a diode, it will not conduct, it might as well not be there at all.

So, first look at D3. At which voltage will it start conduction? Which voltages at u1 are impossible because D3 would (over-) conduct?

Now apply the same logic to D2. It will start conduction when u1 is below a certain voltage.

Now you have the range of possible voltages at u1. Check your list of input voltages against this range.