What effect did increasing the extracellular potassium have on the resting membrane potential?

The resting membrane potential became less negative.

Explanation:

Resting Membrane Potential is defined as the difference in charge or voltage across a cell membrane when the cell is at rest.

It is is also the product of the distribution of charged particles (ions).

However when the extra cellular potassium is increased, the resting membrane potential becomes less negative.