Question

insert one comma to separate the coordinate adjectives.
,
photographs taken by enormous weather satellites help meteorologists forecast
the weather and monitor threatening unstable conditions.

Explanation:

Coordinate adjectives are adjectives that modify the same noun and can be separated by a comma (and also can be reversed in order or have "and" between them). In the phrase "threatening unstable conditions", "threatening" and "unstable" both modify "conditions" and are coordinate adjectives. So we insert a comma between them.

Answer:

Photographs taken by enormous weather satellites help meteorologists forecast the weather and monitor threatening, unstable conditions.