Final answer:
Claude Monet's focus in the painting 'Rouen Cathedral: The Portal (Sunlight)' was on capturing the changing effects of light and color on the surface of the cathedral, particularly the way shadows and light interact on the façade.
Step-by-step explanation:
In the series of paintings, including Rouen Cathedral: The Portal (Sunlight) by Claude Monet, the artist focused on capturing the changing effects of light on the surface of the cathedral over different times of the day. Monet was particularly interested in how the light and color transformed the façade. He used short brushstrokes to define the sculpted stone, which could appear blue-gray in the early morning or bright oranges and yellows when the sun shone directly on the façade. His paintings, including the ones where the front of the cathedral is depicted at an angle, highlight these transient effects of light rather than any particular element like the spires or the door, thus answering the question, the focus was primarily on the content of the shadows of the facade's deep niche.