Combining quotation marks with other punctuation

Which is correct?
According to Peter Drucker “there is only one valid definition of business purpose: to create a _________
a. customer.”
b. customer”.

Explanation:
In the U.S., periods (and commas) always go inside quotation marks; in the U.K., they usually go outside.

Writers Block explains the finer points of U.S. and British differences. Here’s an excerpt:

In the American style, periods and commas are always placed inside the quotation marks, for typographical reasons. In the British style, periods and commas are placed inside the quotation marks only when they are part of the quoted material, which is the more logical placement.

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