The customer can access local resources (a database server), which means local networking is working.
However, the inability to reach the internet suggests an issue with the default gateway. If the default gateway is unreachable, packets will not be routed outside the local network.
Breakdown of Options:
A). Incorrect DNS - DNS issues would cause problems resolving domain names, but the user should still be able to access external resources via IP addresses.
B). Unreachable gateway - # Correct answer. If the default gateway is incorrect or unreachable, the device cannot route traffic to the internet.
C). Failed root bridge - STP (Spanning Tree Protocol) failures cause switching issues, but the user can still access local devices, meaning STP is not the problem.
D). Poor upstream routing - Would affect the entire network, not just one user.
Reference:
CompTIA Network+ (N10-009) Official Study Guide - Domain 2.3: Explain network routing concepts.