A reliable website/application will be the one which
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is likely to be fault-free
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is unlikely to cause a failure
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is likely to be liked by the users
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is unlikely to be completed on schedule
In software testing terminology, reliability refers to the likelihood that a system will operate without failure under specified conditions for a given time. 'Unlikely to cause a failure' (Option B) correctly captures this - reliability is about failure-free operation, not being fault-free (A), user popularity (C), or schedule adherence (D). A system can be reliable even if it contains faults, as long as those faults aren't triggered.
Reliability is defined in terms of failure, not defects: a reliable system is one that is unlikely to fail during operation, i.e., it behaves as expected under real usage conditions. A system can contain latent faults (be 'not fault-free') yet still be highly reliable if those faults are never triggered in practice, so 'fault-free' isn't the defining property. Being liked by users or missing schedule deadlines relate to usability and project management, not reliability.