Geneticists are able to use specific principles to explain the observations made during genetic studies. what can geneticists use the principles of probability for?

Probability is what chance something has to happen. The Punnet Square is a way how to predict in genetics how likely an offspring is to have a trait passed on from parents, or in other words find out the probability of a trait being in the phenotype or the phenotype.
Every enzymatic transaction, every diffusion event, every cellular process only has a certain probability of working. Some of them are very low, but as long as they happen sometimes it's enough for the cell to operate. Thus the cell works around the clock to maintain certain concentrations of certain substrates. 
This occurs with polymerases, the large and small components of ribosomes, even dNTP's. The cell can ensure that a process will occur even though it will likely fail 90% of the times molecules have a chance to interact as long as there are enough molecular substrates present.

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.