The problem states that Google Cloud applications need to access external web services and requires the ability to monitor, control, and log this access.

Monitoring, Controlling, and Logging external web access: This specifically points to a proxy solution, which can intercept, inspect, and log HTTP/S traffic.

Secure Web Proxy (SWP): Google Cloud's Secure Web Proxy is designed for exactly this use case. It acts as an explicit forward proxy for HTTP(S) traffic, allowing organizations to implement granular access controls, inspect traffic for security threats, and log all outbound web requests from their Google Cloud environment.Extract Reference: "Secure Web Proxy is a managed service that lets you deploy and manage an explicit forward proxy to protect your organization's internal resources from web-based threats and to control access to external web applications." and "With Secure Web Proxy, you can: Enforce granular access policies based on different attributes, Log all HTTP(S) requests that are handled by the proxy, and Monitor web traffic for threats." (Google Cloud documentation: https://cloud.google.com/secure-web-proxy)

Let's evaluate the other options:

A. Configure VPC firewall rules to allow the services to access the IP addresses of required external web services: VPC firewall rules operate at Layer 4 (TCP/UDP) and Layer 3 (IP). While they can allow or deny traffic to specific IP addresses and ports, they cannot monitor, control, or log HTTP/S requests at the application layer. They don't provide granular control over which web services are accessed or inspect the content of the requests.

C. Configure Google Cloud Armor to monitor and protect your applications by checking incoming traffic patterns for attack patterns: Google Cloud Armor is primarily a Distributed Denial of Service (DDoS) protection and Web Application Firewall (WAF) service. It focuses on protecting applications from incoming threats (ingress traffic), not controlling and logging outgoing access to external web services.

D. Set up a Cloud NAT instance to allow egress traffic from your VPC: Cloud NAT allows instances without external IP addresses to connect to the internet. While it enables egress, it does not provide monitoring, control, or logging capabilities for specific web services at the application layer. It's a network address translation service, not an application-layer proxy.

Therefore, setting up a Secure Web Proxy is the most appropriate solution to meet the requirements of monitoring, controlling, and logging access to external web services from Google Cloud applications.