What characteristics of beets make them useful as experimental models for studying cellular membranes?

 Since the pigment within the membrane of the beets are colored red, it's good to test the stress caused on the membrane. The more damage to the membrane, the more color it gives off. Put the beet in the solution that you are going to test in. 

Personally, I tested heat and acid. 
In general, extreme heat will cause more stress on the membrane than extreme heat. This can be easily observed by the naked eye. 
Acetone has a greater ability to dissolve lipids than methanol, the acetone solvent will cause greater stress on the membrane.

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option a is correct i.e.

only sodium ions can pass through the membrane.

explanation:

in context to the membrane of nerve impulse, the outer membrane of neuron is semi permeable that is only sodium ions can pass through the membrane i.e. smaller ions can pass through the membrane while larger ions cannot pass through it.