Somebody, please call the Grammar Police and report there has been a murder. Actually, multiple murders. Hundreds of thousands of victims are lying all over the Internet.
The victim is that valuable, essential, basic, explicit word known as “which.” The perp, who hides behind the generic name “what,” is smothering his victims.
Here is a snap shot from just one crime scene (viewer discretion advised; for mature audiences only). These three questions are from the web site of an online payment processor. We have XXXXed its name to protect the guilty.
WHAT BANKS WORK WITH XXXX?
WHAT COUNTRIES CAN I USE XXXX IN?
WHAT PHONES DOES XXXX WORK ON?
The answer to each question refers to a specific, defined set that has a numerical count value. The number of banks, countries and phones, while fluctuating, is a finite, countable quantity. To upstanding, moral, clear-thinking Americans, the accurate word to start those sentences would be “which.”
Those questions are not asking, for example, “What do we call our service?” or “What must I do to use the service?”
These two questions are open-ended. The service provider may (and can, and do) call the service anything. There is no countable limit to either answer. Each sentence is a valid use of the word “what.”
Somehow, the word “which” has become as insulted and ignored as words such as shall, datum and whom.
You might ask, in what year did this happen?
And you would be wrong.