Final answer:
An attacker primarily uses DNS for network foot-printing, which is a technique used to gather detailed information about a network's structure and resources. Therefore the correct answer is Option A.
Step-by-step explanation:
One of the primary ways in which an attacker uses the Domain Name System (DNS) is A. Network foot-printing. This involves gathering information about a network's structure and resources, which can include mapping out domain names, IP addresses, and network services. Attackers may use this information to plan further attacks or breaches. DNS can be exploited in various ways, such as performing DNS reconnaissance to gain information on the network or using DNS spoofing to redirect traffic to malicious sites. While other options like B. Network sniffing, C. Database server lookup, and D. Registration counterfeiting also represent potential security threats, they do not primarily involve the use of DNS by attackers as much as network foot-printing does.