ANS:C - not less than 6

As per IS 456:1978, which is an Indian Standard code for the design and construction of reinforced concrete structures, the pH value requirement for water used in concrete construction is specified to ensure the durability and quality of concrete. The relevant clause in IS 456:1978 states that: "The pH value of water used for mixing and curing shall generally be not less than 6." Therefore, according to IS 456:1978, the pH value of water used in concrete construction should not be less than 6. This requirement is important because acidic water can adversely affect the properties of concrete, such as setting time, strength development, and long-term durability. Ensuring that the pH value of water is not less than 6 helps to mitigate the risk of such detrimental effects on concrete quality and performance.