The correct statement is Linking issues requires a project permission called "Link Issues".

This is true because creating or removing links between issues is governed by the project permission scheme and users must be granted the Link Issues permission for the particular project to perform linking actions.

Outward link description and inward link description must be different names is incorrect because link types define an outward and an inward description but they do not have to be different text and the system will accept identical or similar descriptions if desired.

Issue linking cannot be turned off from the global administration settings is incorrect because administrators can control linking behavior through permissions and global administration options or by changing the available link types, so it is not true that linking can never be disabled or restricted.

Only site administrators can define new types of issue links in the instance is incorrect because defining link types is a global administration capability and can be done by users with the appropriate Jira administration or system administration rights, not strictly by a site administrator role alone.

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