Retrospective application of a change in accounting principle is impracticable when
1. the costs of applying the new accounting principle to prior period financial statements are material.
2. retrospective application requires that management's intent in a prior period be assumed without independent substantiation.

C. Proofreading

Explanation:

Option A is wrong. In an instant message, no one is trying to create a content outline as it is not a letter or assignment.

Option B is incorrect. An instant message can be sent to many people at a time. Therefore, maximizing the number of receivers is the main idea while writing an instant message.

Option D is also not correct. Acronyms are hard to understand. Writing acronyms in an instant message will be challenging for the receivers.

Option E cannot be the answer as an instant message can be sent anytime. Therefore, it can be increased.

Option C is correct because while writing an instant message, proofreading is a necessary stage. If anything misspelled or miswritten, the explanation will change.