The image shows two identical rectangles: one is on the right side of the y-axis (labeled ABCD) and the other is on the left side (labeled A'B'C'D'). The transformation involves the movement of the rectangle ABCD to A'B'C'D'.

To identify the transformation, let's analyze the situation:

1. The two rectangles appear to be mirror images of each other across the y-axis, which suggests a reflection transformation.
2. The reflection is happening across the y-axis specifically, not across another diagonal or slanted line.

Based on this analysis, the transformation applied is a reflection across the y-axis.

So, the correct answer is:

b. Refleksi terhadap sumbu y (Reflection across the y-axis).

Would you like further details or have any questions about this problem?

Here are some related questions to explore further:

1. How do you determine the line of reflection for a geometric figure?
2. What are the properties of reflection transformations in the coordinate plane?
3. What would the coordinates of points A, B, C, and D become after reflection across the y-axis?
4. How does a reflection across the x-axis differ from one across the y-axis?
5. What is the rule for reflecting a point across the line y = -x?

Tip: When reflecting a figure across the y-axis, the x-coordinates of the points change sign, but the y-coordinates remain the same.