Which of the following forms of purchasing power parity states that changes in spot rates over a period of time reflect the changes in price levels over the same period in the currencies of the concerned economies?

The correct answer is Relative form.

Key Points 

•   The relative form of purchasing power parity (PPP) states that changes in spot exchange rates between two currencies over a period of time reflect the changes in relative price levels between the two economies over the same period.
• In other words, if one country's inflation rate is higher than another's, its currency should depreciate relative to the other country's currency to maintain purchasing power parity.
• This theory is based on the idea that exchange rates should adjust to equalize the purchasing power of different currencies.

Additional InformationContango form- Contango is a term commonly used in the commodities market to describe a situation where the futures price of a commodity is higher than the expected spot price at the delivery date. This situation typically arises when the cost of carrying or storing the commodity between the contract date and the delivery date is factored into the futures price. 

Hence, the correct answer is Relative forms.