Security challenges posed by the communications between layers in a client/server environment are:

In a client/server environment, the communication between layers is crucial for the functioning of applications, but it also presents specific security challenges. The correct answer accurately identifies the types of threats that can exploit vulnerabilities in those communications.

Tapping refers to the unauthorized interception of communication, where an adversary listens in on the data being transmitted between the client and server. This can occur over various media, including internet connections and local networks. Sniffing is a specific technique often used to capture data packets as they travel through the network, allowing attackers to glean sensitive information like passwords or confidential messages. Message alteration involves modifying data in transit, which can lead to unauthorized actions or incorrect information being processed, impacting the integrity and reliability of the communication.

These threats emphasize the importance of securing data in transit, typically through encryption and proper authentication methods, to protect against such vulnerabilities effectively. The other options mentioned present security issues but do not align as closely with the specific challenges inherent in the client/server communication layers.