What
happens once a neurotransmitter is received by a post synaptic neuron receptors

Neurotransmitter binding increases membrane permeability and form a new action potential.

Explanation:

Neurotransmitters are specific lipid-based compounds that are used in the nervous system in chemical signal transduction. They exhibit receptor-specific functions in triggering many regions and physiological responses of the brain and spinal cord or the central nervous system (CNS).

When released into the space between the synapses called the synaptic cleft, they trigger a range of responses. On the postsynaptic cells  they briefly bind to chemical receptors. This results in changes to the membrane which increases the cells' permeability to ions like Na+; a new action potential is created.

a polymer is a chainlike molecule made up of small molecular units. these small molecular units are called monomers. monomers of polymer can be joined together either naturally or synthetically.  

c. earths continental crust is thicker than its lithosphere.

because, the lithosphere is the outermost layer of the earth/ 100 km thick and is defined by its machanical properties.the lithosphere contains oceanic and continental crust that varies in age and thickness across locations and geologic time. the lithosphere is the coolest layer of the earth in terms of temperature. the term lithosphere should not be confused with the use of "geosphere" which is used to indicate all of earth systems, including the atmosphere, hydrosphere   and biosphere. the surface of the earth can be divided into the continental crust and the oceanic crust.   the continental crust is made mostly of granite while the oceanic crust is made of basalt. the average thickness of the continental crust is 25 miles and the average thickness of the oceanic crust is 5 miles.   three types of rock, igneous, sedimentary, and metamorphic, make up the surface of the earth. that's why 3 option is preferable.