Final answer:
The ISO 9126 standard defines software quality factors, and the correct factors are functionality, portability, and reliability.
Step-by-step explanation:
The ISO 9126 standard defines six software quality factors: functionality, reliability, usability, efficiency, maintainability, and portability. Visual appeal is not included as one of the ISO 9126 software quality factors.
Functionality refers to the software's ability to provide the intended features and meet the user's requirements. Reliability measures the software's ability to perform consistently and correctly under different conditions. Portability refers to the software's ability to be easily transferred or adapted to different environments or platforms.
Therefore, the correct answer is option 5: a, b and c, which includes functionality, portability, and reliability as ISO 9126 software quality factors.