A solution of ________ is formed when the white ash formed by burning magnesium ribbon is dissolved in water.

The correct option is B magnesium hydroxide
Magnesium ribbon is burnt in air to form the white ash of magnesium oxide. This can be represented as:

Magnesium + Oxygen $\to$ Magnesium oxide

Magnesium hydroxide is formed when the white ash of magnesium oxide is dissolved in water. This can be represented as:

Magnesium oxide + Water $\to$ Magnesium hydroxide