Answer:
3.co-release and cotransmission of more than one neurotransmitter
Step-by-step explanation:
The complete question is as follows:
The discovery of what properties of neurotransmission caused neuroscientists to rethink Dale's Principle?
- inhibition of neural activity
 - neural communication at more than one place at a neuron
 - co-release and cotransmission of more than one neurotransmitter
 - finding similar neuronal activity in more than one location in the brain
 
- Dale's principle was given by a scientist named Henry Dale in 1940's and the principle state that each neuron is capable of releasing only a single type of neurotransmitter.
 - This principle meant that a particular neuron is capable of performing only a single function at all the synapses.
 - However, Dale's principle were soon proven to be false when it was discovered that there are many neurons capable of releasing more than one neurotransmitter at a time and this is termed as cotransmission and co-release of the neurotransmitters. 
 - An example of co-transmission is the release of GABA along with substance P as cotransmitter.