The correct answer is option 3: agriculture was prevalent in Harappan sites. This means that farming and cultivation of crops was common in the Harappan civilization. The prevalence of grain in Harappan sites suggests that the people of this civilization relied on agriculture as a means of sustenance.

Option 1 states that Harappans ploughed their fields. Ploughing is a common farming practice that involves turning over the soil with a tool called a plough. However, the given information does not directly indicate whether or not the Harappans used ploughing as a method of farming.

Option 2 suggests that Harappans broadcast seed on their fields. Broadcasting is a method of sowing seeds by hand, scattering them evenly across the field. While it is a possible farming method, the given information does not specifically mention this as the primary farming practice of the Harappans.

Option 4 states that trade in bulk items like food grains was well-established in Harappan times. While trade and commerce were indeed significant in the Harappan civilization, the given information does not specifically indicate that the prevalence of grain was solely due to trade.