Consider a multi party system where public keys of all receivers are known. Server wants to send message to one of them using (textbook?) RSA. Adversary is given ciphertext and even Message to be send, and all $k$ public keys of participants. Can any attack tell who is the target receiver?
Or in nutshell: Given $C$, $M$ and all $k$ public keys, can an attacker tell with significant probability which public key was used to encrypt $M$ giving $C$ ?
Editor's note: The textbook RSA part is per OP's comment. It was originally plaintext RSA. Alternatively, the question makes (more?) sense with some random encryption padding.