d
Potential difference between $A$ and $B$
${V_A} - {V_B} = 1 \times 1.5$
$ \Rightarrow \,\,\,{V_A} - 0 = 1.5\,V \Rightarrow {V_A} = 1.5\,V$
Potential difference between $B$ and $C$
${V_B} - {V_C} = 1 \times 2.5 = 2.5\,V$
$ \Rightarrow 0 - {V_C} = 2.5\,V \Rightarrow {V_C} = - 2.5\,V$
Potential difference between $C$ and $D$
${V_C} - {V_D} = - 2V$$ \Rightarrow \,\,\, - 2.5 - {V_D} = - 2 \Rightarrow \,{V_D} = - 0.5\,V.$