What governs whether copper water pipe may connect directly to galvanized steel pipe without additional protection?
Not quite. The answer is A: A dielectric fitting or union (or another approved transition fitting) is used to separate the dissimilar metals
Copper and galvanized steel in direct contact, with water present, form a galvanic cell that corrodes the more reactive metal at the joint. For a copper-to-threaded-pipe connection the code calls for a copper alloy adapter, a copper alloy nipple of at least 6 inches, or a dielectric fitting or union, and dielectric unions are specifically called out wherever dissimilar metals meet — so a bare metal-to-metal joint is not simply allowed.
Checked against the 2021 UPC text. Washington's exam is written against the 2015 edition, which we have not read for this section, so confirm it against your 2015 book.
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