In the initial situation (12:00Z) you see a classic polar-front wave cyclone:

• Cold front (blue) advancing from the southwest
• Warm front (red) ahead to the east

By 16:00Z, the faster-moving cold front has overtaken the warm front, forming an occluded front near the low centre. The type of occlusion is determined not by the warm air, but by comparing the temperatures (and densities) of the cold air masses ahead of and behind the system.

In Chart Y, the air behind the cold front is colder and denser than the air ahead of the warm front. As the cold front catches up, it undercuts both the warm air and the relatively milder cold air in front of the low. This is the defining characteristic of a cold occlusion. 

Chart Z, on the other hand, represents a warm occlusion, where the cold air ahead of the system is colder and denser than the air behind. That scenario does not fit the synoptic evolution shown here.

• Cross-section 1 illustrates a cold occlusion. The coldest air behind the front wedges underneath, forcing both the warm air aloft and the less cold air ahead of the depression to rise.
• Cross-section 2 shows a warm occlusion, in which the warm air and the less cold air behind the low are lifted over a colder, denser air mass ahead, again inconsistent with the selected chart.

In a cold occlusion, the occluded front usually keeps the same curved shape and movement as the original cold front, which is shown in Chart Y. In a warm occlusion, the occluded front looks more like the warm front, as seen in Chart Z. An occlusion forms because the cold front moves faster than the warm front, so it eventually catches up with it near the centre of the low-pressure system. In this situation, the temperature pattern and the way the fronts move clearly show a cold occlusion, which means Chart Y and cross-section 1 are the correct choice.