These six adjectives overlap, but they emphasize different things. Your choice depends on whether you mean a clear change, a strong quality, something that stands out, or something that gets in the way.
- Noticeable means easy to notice. It does not necessarily suggest anything dramatic. There was a noticeable scratch on the table. The scratch was visible, but it might have been small.
- Marked emphasizes a clear or significant difference, change or characteristic. There was a marked improvement in her reading. This suggests more than a minor improvement.
- Pronounced means very noticeable or strongly developed. It often describes features, qualities or differences. He speaks with a pronounced accent. The accent is especially strong or clear.
- Conspicuous describes something that stands out and attracts attention. Her orange coat was conspicuous in the crowd. The attention need not be unwelcome.
- Prominent can mean easy to see because of position or appearance: The notice occupied a prominent position by the entrance. It can also mean important or well known: She is a prominent scientist.
- Obtrusive suggests unwanted attention or interference. The flashing advertisement was obtrusive. It distracted people rather than simply being visible.
These are not fixed steps on a scale. A difference can be both marked and pronounced. A sign can be prominent without being obtrusive: being easy to see does not automatically make it annoying.