The fact that current is uniform anywhere in a series circuit is an application of the principle of:

newton's second law

conservation of charge

conservation of energy

coulomb's law

In series circuit there is no branching of the resistances

so here we know that all the resistors are connected without any branching and current will flow through all branches without any loss.

So here we know that current is defined as rate of flow of charge

now here we can see that charge Q will flow at same rate through all the resistors which are connected in series

so here

so this will show the conservation of charge

so correct answer will be

conservation of charge