Questions: Chalk forms from - magnesium rich fluids that chemically alter limestone bearing reefs - evaporation of magnesium rich waters - the hard parts of microscopic organisms that accumulate on the sea floor - direct precipitation from seawater

The answer is the third one: the hard parts of microscopic organisms that accumulate on the sea floor.

Explanation for each option:

1. Magnesium rich fluids that chemically alter limestone bearing reefs: This process is more related to the formation of dolomite rather than chalk. Dolomitization involves the replacement of calcium in limestone with magnesium.

2. Evaporation of magnesium rich waters: This process typically leads to the formation of evaporite minerals such as gypsum and halite, not chalk.

3. The hard parts of microscopic organisms that accumulate on the sea floor: This is the correct answer. Chalk is primarily composed of the calcium carbonate (CaCO₃) remains of microscopic marine organisms such as foraminifera and coccolithophores. These organisms' hard parts accumulate on the sea floor and, over geological time, compact and cement to form chalk.

4. Direct precipitation from seawater: While calcium carbonate can precipitate directly from seawater, this process typically forms different types of limestone, not chalk.

Summary: Chalk forms from the hard parts of microscopic organisms that accumulate on the sea floor.