HTTPS (Hypertext Transfer Protocol Secure) is the best choice for secure client-server communications involving sensitive data because it provides a secure version of HTTP by incorporating SSL/TLS to encrypt the data in transit. This ensures that all data sent between the client and server is encrypted. In contrast, LDAP, while used for accessing and maintaining distributed directory information services, does not inherently encrypt data. Multicast DNS (mDNS) resolves host names to IP addresses within small networks and does not involve encryption or secure data transactions. SNMP, used for network management, is also not suitable for secure transactions as it primarily facilitates monitoring and not secure data exchange.