Synaptic transmission is terminated by a- block of pretsynaptic receptorsb- elevation of Ca ++ concentration in synaptic cleftc- reuptake of neurotransmitters by postsynaptic neuronsd- degradation of neurotransmitters by specific enzymes

Synaptic transmission is primarily terminated by two mechanisms:

1. Reuptake of neurotransmitters by presynaptic neurons: After a neurotransmitter has been released into the synaptic cleft and has bound to the postsynaptic receptors, it needs to be removed to prevent continuous activation of the postsynaptic neuron. This is primarily achieved by reuptake, where the neurotransmitter molecules are reabsorbed back into the presynaptic neuron. This is a common mechanism for neurotransmitters like dopamine, norepinephrine, and serotonin.

2. Degradation of neurotransmitters by specific enzymes: Some neurotransmitters are broken down by enzymes in the synaptic cleft. For example, acetylcholine is broken down by the enzyme acetylcholinesterase. The products of this enzymatic breakdown can then be reabsorbed by the presynaptic neuron and used to recreate the neurotransmitter.

Therefore, the correct answer is c- reuptake of neurotransmitters by presynaptic neurons and d- degradation of neurotransmitters by specific enzymes.

Blocking of presynaptic receptors (a) and elevation of Ca++ concentration in the synaptic cleft (b) are not primary methods of terminating synaptic transmission. Blocking of presynaptic receptors would prevent the initiation of a new signal, but would not terminate an ongoing one. Elevation of Ca++ concentration in the synaptic cleft is actually part of the process that triggers the release of neurotransmitters from the presynaptic neuron, so it would not serve to terminate the signal.