Final answer:
The primary purpose of a standard for creatives to run across multiple ad networks and websites unmodified is to reduce creative development costs by avoiding the need to create different versions for each platform.
Step-by-step explanation:
The primary purpose of a standard that enables a single creative to run unmodified across multiple ad networks and websites is C) To reduce creative development costs. This standardized approach allows advertisers to build a single version of an ad that can be distributed across various platforms without needing alteration for each one, which in turn reduces the time, resources, and expenses associated with creating multiple versions of the same ad for different specifications and formats required by various ad networks or sites. Saving on these costs can be particularly important for businesses that are aiming to optimize their advertising budgets and enhance their overall marketing efficiency.