Don’t leave your car unlocked

Don’t leave your car unlocked, said my friends. My friends warned me ............. my car unlocked .
a)to not leave - b)about leaving - c)about not leaving - d)none

Source: school exam

Hello,
I have asked this question in two different forums and received different answers.

Some say only B is correct.
And some other say A, B, C all can be correct because:
(warn + somebody (not) to do something) (warn somebody about something)
If your friends warn you, they are warning you against danger, against something bad - something that could mean your car is stolen.
Option C does not work because nobody warns somebody about NOT leaving a car unlocked.
Will your friends warn you about not leaving your car unlocked? That is not the danger. There is no danger in not leaving your car unlocked.
My friends warned me about leaving my car unlocked. = I shouldn't leave my car unlocked.
My friends warned me about not leaving my car unlocked. = I should leave my car unlocked.

Would you please be kind enough to give me some guidance on this question? Who do you agree with?

Thank you.