When a user is browsing a website on a smartphone, a pop-up appears.

Comprehensive and Detailed In-Depth Explanation:

This is a scareware scam that attempts to trick users into installing malicious applications. The best action is to close the website and ignore the pop-up.

B. Replace the smartphone – Incorrect. The phone is likely not compromised; it is just displaying a fake warning.

C. Download and install the application to resolve the issue – Incorrect. Doing so could infect the device with malware.

D. Analyze the smartphone to find the source of the breach – Unnecessary, as there is no real breach.