This sentence makes no sense in German. Deutlich is always used in the context of speaking clearly (or not). E.g. "Deine Aussprache ist deutlich", " Der Mann spricht nicht deutlich".
Maybe he has done the exercise on the PC where you don't have words to choose from.. Or directly on the cell browser. The type of exercise dipends on the kind of duolingo you are using.
This is odd usage in English, where any number of adjectives could have worked better: plain, elegant, meticulous...
It didn't give me the choice of clear, so I chose articulate. It was accepted but sounds weird.