Answer:
Dredging means "to scooping out dirt"
Step-by-step explanation:
Dredging is defined as the service of removing dirt and impurities from the bottom of rivers, lagoons, seas, bays and port access channels. With this conceptualization we can understand what the grandfather of the speaker of the paragraph in the above question did. He helped clean the city lake in the 1960s so that children could swim.
From this we can conclude that the correct answer to your question is "to scooping out dirt".