An ideal current source delivers a constant current to a load. For a real energy source to behave as a constant-current source, its internal resistance should be:

very large compared to the load resistance
equal to the load resistance
zero
non-zero but smaller than the load resistance

Correct answer: very large compared to the load resistance

Solution

When the internal resistance r is much larger than the load resistance R, the current I = EMF/(r+R) ~= EMF/r stays essentially constant regardless of R.

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