When fresh milk turns sour, the pH value will decrease. This means that the pH will become less than 6. The given options can be dissected as follows:

Option 1: Becomes < 6 - This option is correct as the pH of sour milk will be lower than 6.

Option 2: Remains the same, i.e., 6 - This option is incorrect as the pH of sour milk is expected to decrease.

Option 3: Becomes > 6 - This option is incorrect as the pH of sour milk is not expected to increase.

Option 4: Becomes neutral, i.e., 7 - This option is incorrect as the pH of sour milk is not expected to become neutral.

In conclusion, when fresh milk turns sour, the pH value becomes less than 6. It is important to note that the decrease in pH is due to bacterial fermentation of lactose into lactic acid, which contributes to the sour taste and lower pH of sour milk.