The correct options are
A They are effective when data is aggregated into enumeration units.
C They can be standardised to show ratios.
D They can be used to measure continuous statistical fractions.
Choropleth maps use sequential colour combinations to depict classified data. They can be used when the data is attached to enumeration units like districts, countries, provinces, etc. They are best suited for raw data. They can also be used effectively when data is standardised into different ratios, rates, or percentages.