For a relation to be a partial order relation it must be Reflexive, Anti symmetric and Transitive.
For a relation to be transitive it must follow transitive property.
Transitive property: If a R b and b R c then a R c. Where a R b means a is related to b.
In A graph, a R b and b R d but a is not related to d. It is violating the transitive property. Hence it is not Partial order relation.
In B graph, c R d and d R b but c is not related to b. It also violated the transitive property.
For C graph,
As every vertex has Self loop, indicating that graph is reflexive.
It is anti symmetric because whenever a R b and b R c imples (a = b).
It is transitive also. because there is no case where x R y and y R z. So x R y and y R z become false. Whenever in P $\rightarrow$ Q , P become false, P $\rightarrow$ Q become True.
So C graph is Partial order relation.
Ans: D.