Brown eyes (b) are dominant over blue eyes (b) and straight tails (t) are dominant over curly tails (t) in opossums. a brown-eyed, straight-tailed opossum male was mated to a tester female. the offspring were: 3 brown eyes and straight tails 2 brown eyes and curly tails 4 blue eyes and straight tails 3 blue eyes and curly tails what was the genotype of the male?

When there is a tester mate it means that the genotype of that mate is known, most commonly that mate has all the recessive traits that are being studied so that we know that mate's genotype is homozygous recessive.
Therefore, in this case the tester female has probably blues eyes and curly tail, meaning that the only contributed combination of alleles from the tester mate is blue eyes and curly tail. As we can see by the results of the mating, the male would have to have all types of alleles. Because the male has a brown-eyed and straight-tailed phenotype it means that he has at least one allele that's for brown eyes and one that's for straight tail. Once the offspring includes all types of phenotypes, we can conclude that the male had also the recessive alleles for both characteristics. 

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.