The most likely answer is a) calcium. Calcium oxide (CaO), which is the compound formed when calcium reacts with oxygen, has a high melting point and is soluble in water.
Here’s why the other options are less likely:
b) carbon: Carbon dioxide (CO2) has a relatively low melting point and is not very soluble in water.
c) silicon: Silicon dioxide (SiO2), also known as quartz or sand, has a high melting point but is not soluble in water.
d) iron: Iron oxide (Fe2O3) has a high melting point but is not soluble in water