The correct answer is:

Loss or corruption of data is almost always non-deliberate.

and

It is possible to prevent all threats cost-effectively.

Rationale: Data can be lost or corrupted as the result of deliberate actions such as fraud, sabotage, commercial espionage or malicious damage - as well as human error.

It is impossible to prevent all threats (important to be realistic!); the other statements are true: new staff pose a security risk because of the risk of human error (if they are inexperienced or untrained); and smoke detectors are an example of protection from physical risks (in this case, fire).

Pitfalls: Don't forget physical security measures: you may have discounted the smoke detectors option too quickly. Data security also involves protection against fire, flood, interruption of power source and so on.