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taskmasters
The answer to the question stated above is letter a. diatoms.
"Diatoms are unicellular algae that has cell walls made of silicon."

Diatoms are plantlike protists (unicellular algae) that produce thin, delicate cell walls rich in silicon; These planlike protists are members of the phylum Bacillariophyta.
JcAlmighty
The answer is a. diatoms.
Diatoms are unicellular algae. They are specific for their glass-like cell walls with sculpturing. Their cell wall is made of hydrated silicon dioxide. The cell wall is rigid and transparent because of silicon dioxide. Silicon cell wall cannot decompose, so can be a great help in explaining past conditions.