The correct answer is C. Insecure Data Storage because the vulnerability described is the storage of sensitive information locally on the mobile device in a manner that is not encrypted and is accessible by simply browsing the file system. In mobile application security, this is a classic risk category: when an app stores confidential data (case notes, client records, tokens, documents, cached responses, databases, logs, or exported files) in clear text or in insecure locations, an attacker who gains device access—or uses backup extraction, file explorers, rooted/jailbroken access, or malware with storage permissions—may retrieve that data without needing to authenticate to the application.

The scenario makes the weakness unmistakable: Daniel can use a “standard explorer tool” to open sensitive records “without any authentication.” This indicates the app is failing to apply appropriate protections such as encryption at rest, secure key handling, proper file permissions, and secure storage mechanisms. In secure mobile design, sensitive records should be encrypted using platform-supported protections (e.g., using OS keystores/keychains for keys, encrypting databases/files, and minimizing local retention). Additionally, apps should avoid storing highly sensitive regulated data unless essential, and should implement secure session controls and data lifecycle management (cache control, expiration, remote wipe support in enterprise settings).

Why the other options are not the best fit: Insecure communication concerns data exposure while transmitted over networks (e.g., lack of TLS, weak TLS, MITM susceptibility), whereas the issue here is purely local storage. Improper credential usage relates to mishandling passwords, tokens, or authentication secrets (hard-coded credentials, weak storage of credentials), but the prompt focuses on stored records themselves. Inadequate privacy controls is broader and typically involves over-collection, improper disclosure, or weak user privacy settings, not direct clear-text storage exposure.

Therefore, the most clearly present OWASP Top 10 Mobile Risk is Insecure Data Storage.