Final answer:
The abbreviation 'LOL for Laugh Out Loud' is considered informal and often used in casual digital communication, whereas formal writing requires full names before abbreviations, such as 'USDA' or 'OSHA', and consistent abbreviation conventions like 3.5 mL for milliliters.
Step-by-step explanation:
The abbreviation considered informal among the choices given is LOL for Laugh Out Loud. This type of abbreviation is commonly used in casual digital communication such as texting and social media. In formal writing, it is important to write out full names the first time you use them, followed by abbreviations or acronyms in parentheses. For instance, you would write U.S. Department of Agriculture (USDA) or Occupational Safety and Health Administration (OSHA) initially, and then use the abbreviation or acronym for subsequent references. Similarly, in the context of metric abbreviations, while most are lowercase, like 'm' for meter, some are uppercase, such as 'L' for liter, so 3.5 milliliters is written as 3.5 mL.