a. The speaker is the current Receiver of Memory, Jonas.
b. "They" refers to the Elders or the community leaders who have designated Jonas as the Receiver of Memory.
c. "That burden" refers to the memories that Jonas has been receiving from the Giver, which contain both positive and negative experiences from the past, such as love, pain, war, and death.
d. They might have given it to him because they believe that it is necessary for the Receiver of Memory to have knowledge of the past in order to make informed decisions for the community's future. The community has chosen to eliminate emotional depth, pain, and suffering, by eradicating memories of the past, but the Receiver of Memory has the responsibility of holding onto those memories, so that they are not lost forever