Prominent means to standout or project beyond a surface or line. Also readily noticeable.
Merriam- Webster Dictionary
“A Hunger Artist” by Franz Kafka
“-Spurning a chair – in black tights, looking pale, with his ribs sticking out prominently, sometimes nodding politely, answering questions with a forced smile, even sticking his arm out through the bars to let people feel how emaciated he was…”
Now fining out the definition of he word prominent, i understand the meaning of the sentence. The sentence is telling us how his ribs are standing out and being projected.