What does physical custody mean here?

Business English Lesson, Advanced Level

ESL/EFL Test #180 [color=blue]“The US Mint”, question 8

In addition to producing coins, the United States Mint has other responsibilities, including maintaining physical and protection of the Nation’s $100 billion of U.S. gold and silver assets.

(a) custody
(b) ownership
(c) possession
(d) storage

Business English Lesson, Advanced Level

ESL/EFL Test #180 [color=blue]“The US Mint”, answer 8

In addition to producing coins, the United States Mint has other responsibilities, including maintaining physical custody and protection of the Nation’s $100 billion of U.S. gold and silver assets.

Correct answer: (a) custody
[size=200]_________________________[/size]

What does physical custody mean here?
thanks.

Having and holding the actual item.