why not other why head is correct?

[Leonidi Karot]

English Language Tests, Intermediate level

ESL/EFL Test #288 "English Slang Idioms (10)", question 2

He studied the reason behind the issue for years and when he explained it to his coworkers, they all agreed that he knew what he was talking about. He really hit the nail on the ..........

(a) board
(b) metal
(c) head
(d) hand

English Language Tests, Intermediate level

ESL/EFL Test #288 "English Slang Idioms (10)", answer 2

He studied the reason behind the issue for years and when he explained it to his coworkers, they all agreed that he knew what he was talking about. He really hit the nail on the head.

Correct answer: (c) head
_________________________

why not other why head is correct?

Leonidi Karot

[Mister Micawber]

.
These are IDIOMS being tested. 'Hit the nail on the head' is the idiom for 'identify precisely'.
.
_________________
Native English teacher at Mister Micawber's

[Tom]

And I think once Mister Micawber used this idiom (partially) here: 'it is not impossible' vs. "there's a possibility".

Tom