Which term can be added to the list so that the greatest common factor of the three terms is 12h3? 36h3, 12h6,

Any multiple of 12h^3.

Remember that the greatest common factor is the common terms raised to the lowest power. Then you can include any terms that includes 12h^3

For example, 12h^3, or 24h^12, 60h^9, etc.