Step 1: Identify the shift in language. Diagnostic bodies moved away from the stigmatising label mental retardation and adopted a more respectful, descriptive name.
Step 2: State the accepted term. The currently endorsed name is intellectual disability (also known as general learning disability), defined by deficits in both reasoning ability and adaptive day-to-day functioning that arise during development.
Step 3: Rule out the rest. Feeble mindedness and madness are obsolete lay terms, and mentally unstable is vague and incorrect, so none of these can be the official replacement.
\[\boxed{\text{Intellectual disability}}\]