I think it should be C.
During pregnancy, there is an increase of blood volume causing a physiologic anemia.
A is not the correct option because it may be common during the 1st and 3rd trim due to pressure of the bladder but not Na.
B is not the correct option because heart rate i think increases rather than decreases.
D is incorrect because a pregnant woman may have a shallow respiration due to the inability of the lungs to fully expand (pressure of the enlarging uterus that pushes the uterus up)