An element reacts with oxygen to form oxide with a high melting point. This compound is also soluble in water. The element is likely to be:

The correct option is A Potassium
Potassium react with oxygen to give potassium oxide $\left(K_{2}O\right)$. Potassium oxide is soluble in water to form alkali.
$K_{2}O\left(s\right)+H_{2}O\left(l\right)\to 2KOH(aq.)$
Carbon react with oxygen to form carbon dioxide gas.
Silicon reacts with oxygen to form silicon dioxide.
Iron reacts with oxygen to form water insoluble oxide.