How does radar work?

Question $13$ options:

$1)$

Detecting radio waves reflected by the object being studied.

$2)$

Detecting sound waves reflected by the object being studied.

$3)$

Detecting radio waves emitted by the object being studied.

$4)$

Detecting sound waves emitted by the object being studied.

$5)$

Measuring the time delay between successive radio wave emissions.

$6)$

Measuring the time delay between successive sound wave emissions.