Explanation:

- Elasticity of demand (Ed) measures how the quantity demanded changes in response to a change in price.

- When the demand curve is parallel to the Y-axis, it indicates a perfectly inelastic demand.

- Option 1: Unity – Elasticity is one. It means a 1% change in price leads to a 1% change in quantity demanded. This does not apply to a vertical demand curve.

- Option 2: Zero – Elasticity is zero. It means quantity demanded does not change irrespective of price changes. This applies to a vertical demand curve.

- Option 3: Less than unity – Elasticity is between 0 and 1. Demand changes by a smaller percentage than the price. Not relevant for a vertical curve.

- Option 4: More than unity – Elasticity is greater than 1. Demand changes by a larger percentage than the price. Not relevant for a vertical curve.