Explanation

An ornithologist studies birds; an archaeologist studies artifacts (and past human remains/settlements).

Explanation

Paw is the foot of a cat; similarly, a hoof is the foot of a horse (while lambs/sheep also have hooves, the standard pairing is horse).

Explanation

Flow describes what a river has; stagnant describes what a pool (stagnant pool of water) has.

Explanation

A denizen is one who dwells in a place; similarly, an heir is one who inherits.

Both pairs show a verb and the person who performs or receives the action.

Explanation

A mongoose is an example of a mammal; likewise, granite is an example of a mineral/rock.

Granite is an igneous rock made of minerals like quartz and feldspar, so Mineral is the best match.