Comprehensive and Detailed Explanation From Exact Extract:

Toil-reduction engineering focuses on making the service itself easier to operate. The most direct way to achieve this is through internal automation --- automation built into the service that eliminates repetitive, manual operational tasks.

The Site Reliability Engineering Book, Chapter ''Eliminating Toil,'' states:

''Automation that replaces manual, repetitive operational tasks is the primary mechanism for reducing toil. The most effective form of toil reduction is automation that is integrated directly into the service itself.''

The SRE Workbook reinforces:

''Internal automation contributes directly to service reliability and reduces the operational burden by ensuring that manual tasks are permanently removed.''

Why the other options are not the best answer:

A Continuous delivery pipelines reduce release friction but do not directly remove service-operational toil.

B External scripts and tools help but are less effective and harder to maintain than internal automation.

C Scalable infrastructure reduces linear-scaling toil but does not address broader operational burdens.

Thus, the correct answer is D.

Site Reliability Engineering Book, ''Eliminating Toil''

SRE Workbook, ''Toil Reduction Approaches''