Introduction:
A firewall is a security device that acts as a barrier between an internal network and an external network (typically the internet). It monitors and controls incoming and outgoing network traffic based on predetermined security rules. The firewall is typically installed at the point where the secure internal network and untrusted external network meet.

Explanation:
The correct answer is option 'A', which states that the firewall is installed at the choke point. Here's an explanation of why this is the correct answer:

1. Choke point:
- A choke point refers to a strategic location where control or restriction can be imposed.
- In the context of network security, a choke point is a location where network traffic is forced to pass through a single point of control or monitoring.
- The firewall acts as a choke point by intercepting and analyzing all network traffic passing between the internal and external networks.

2. Secure internal network:
- The internal network refers to the network within an organization or a specific area that needs to be protected from external threats.
- This network is considered secure as it contains sensitive and confidential information that should not be accessible to unauthorized users.
- The firewall is installed at the choke point to protect the secure internal network from unauthorized access or malicious activities from the untrusted external network.

3. Untrusted external network:
- The external network refers to the network outside the organization or area, typically the internet.
- This network is considered untrusted as it contains potential threats and malicious entities that could compromise the security of the internal network.
- The firewall acts as a barrier between the untrusted external network and the secure internal network, filtering and blocking any malicious or unauthorized traffic.

4. Firewall's role:
- The firewall monitors all incoming and outgoing network traffic based on predefined security rules.
- It examines the source, destination, and content of each packet to determine whether it should be allowed or blocked.
- The firewall can prevent unauthorized access, protect against malware and viruses, and enforce security policies.

Conclusion:
In conclusion, a firewall is installed at the choke point, which is the strategic location where the secure internal network and untrusted external network meet. The firewall acts as a barrier, monitoring and controlling network traffic to protect the internal network from unauthorized access and malicious activities.