Correct option is (a) sodium
The best I can explain: Sodium and potassium are used by the body to conduct impulses but unlike potassium, sodium is a much smaller ion. This means it has higher electropositivity and a higher capacity to attract negatively charged ions. It attracts water molecules and causes water retention, something which is very harmful to a patient suffering from high blood pressure. Thus, their sodium intake is reduced so that the excess water is not held back in blood and the pressure on the heart to circulate the larger volume of blood reduces.