Select all that apply.
cyanobacteria
serve as a food source for other organisms
are used for medicinal purposes
produce oxygen
perform nitrogen fixation
cause disease

Cyanobacteria performs all the function that given in the Question.

Explanation:

Cyanobacteria serve as a food for other organism, They are photosynthetic bacteria. Cyanobacteria and plants are capable of using light energy which trapped by the chlorophyll to convert carbon dioxide and water into oxygen and its food as a glucose, this process is called photosynthesis.

Cyanobacteria are used for medical purpose because they have rich source of biologically active compound with antibacterial, antifungal, antiviral activities.

Cyanobacteria produce oxygen because they oxygenic photosynthetic  bacteria. They trap sun's energy absorbs carbon dioxide and emit oxygen.

Cyanobacteria performs nitrogen fixation because they contain additional fixed nitrogen which absorbs by the plants from their roots. Nitrification cannot occurs in the presence of oxygen. So, nitrogen is fixed iin the specialised cells called heterocysts.

a fossil fuel is a fuel formed by natural processes, such as anaerobic decomposition of buried dead organisms, containing energy originating in ancient photosynthesis.[1] the age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years.[2] fossil fuels contain high percentages of carbon and include petroleum, coal, and natural gas.[3] other commonly used derivatives include kerosene and propane. fossil fuels range from volatile materials with low carbon to hydrogen ratios like methane, to liquids like petroleum, to nonvolatile materials composed of almost pure carbon, like anthracite coal. methane can be found in hydrocarbon fields either alone, associated with oil, or in the form of methane clathrates.

the theory that fossil fuels formed from the fossilized remains of dead plants by exposure to heat and pressure in the earth's crust over millions of years was first introduced by andreas libavius "in his 1597 alchemia [alchymia]" and later by mikhail lomonosov "as early as 1757 and certainly by 1763".[4] the first use of the term "fossil fuel" was by the german chemist caspar neumann, in english translation in 1759.[5]

in 2017 the world's primary energy sources consisted of petroleum (34%), coal (28%), natural gas (23%), amounting to an 85% share for fossil fuels in primary energy consumption in the world.[6] non-fossil sources in 2006 included nuclear (8.5%), hydroelectric (6.3%), and others (geothermal, solar, tidal, wind, wood, waste) amounting to 0.9%.[7] world energy consumption was growing at about 2.3% per year. in 2015 about 18% of worldwide consumption was from renewable sources.[8]

i think its white blood cells

i hope this