To determine which items are not a source of indoor air pollution among the options provided, we can analyze each one:
A. Fireplaces - They can release pollutants, especially if they burn wood.
B. Computers - Generally, they do not produce significant indoor air pollutants but can emit some heat and very low levels of certain chemicals.
C. Cleaning supplies - Many cleaning supplies contain volatile organic compounds (VOCs) that can contribute to indoor air pollution.
D. Space heaters - They can produce indoor air pollutants like carbon monoxide and nitrogen oxides, especially if they are fuel-burning types.
From this analysis, computers (B) are the least significant source of indoor air pollution compared to the others. Hence, the answer is B: computers.