The question is about marking the correct sentences among the given statements related to defects, failures, and mistakes. According to the ISTQB glossary, the definitions of these terms are1:

Defect: A flaw in a component or system that can cause the component or system to fail to perform its required function, e.g. an incorrect statement or data definition. A defect, if encountered during execution, may cause a failure of the component or system.

Failure: An event in which a component or system does not perform a required function within specified limits.

Mistake: A human action that produces an incorrect result.

Therefore, out of the five given statements, only two are correct, namely:

A human mistake may produce a defect: This is true, as a mistake is a source or cause of a defect, e.g. a programmer may make a mistake in writing a code statement, which results in a defect in the software component.

When a defect exists in a system it may result in a failure: This is true, as a defect is a potential or actual cause of a failure, e.g. a defect in the software component may cause the system to fail to perform a required function when the defect is encountered during execution.

The other three statements are incorrect, namely:

Defects are a result of environmental conditions and are also referred to as “Failures”: This is false, as defects are not a result of environmental conditions, but of mistakes or other factors, and defects are not the same as failures, but rather the causes of failures.

A system will totally fail to operate correctly when a failure exists in it: This is false, as a system may not necessarily fail completely or stop operating when a failure occurs, but may continue to operate with reduced functionality or performance, or with incorrect results.

Defects occur only as a result of technology changes: This is false, as defects can occur due to various reasons, not only technology changes, such as human mistakes, design flaws, requirement changes, hardware failures, etc.