Encrypting all sensitive data stored on the device is the best measure to ensure the security of this data, because it protects the data from unauthorized access or disclosure, even if the device is lost, stolen, or compromised. Encryption is a process of transforming data into an unreadable format using a secret key or algorithm. Only authorized parties who have the correct key or algorithm can decrypt and access the data. Encryption can be applied to data at rest, such as files or databases, or data in transit, such as network traffic or messages. Encryption can prevent attackers from stealing or tampering with the customer data stored on the device, such as credit card details and PINs, which can cause financial or identity fraud.

The other options are not as effective or sufficient as encryption for securing the customer data stored on the device. Implementing biometric authentication for app access may provide an additional layer of security, but it does not protect the data from being accessed by other means, such as malware, physical access, or backup extraction. Enabling GPS tracking for all devices using the app may help locate the device in case of loss or theft, but it does not prevent the data from being accessed by unauthorized parties, and it may also pose privacy risks. Regularly updating the app to the latest version may help fix bugs or vulnerabilities, but it does not guarantee the security of the data, especially if the app does not use encryption or other security features.