Yogurt is the product of:

alcohol fermentation
acetobacter
lactic acid fermentation
rennet

Yogurt is the product of Lactic acid fermentation

Explanation:

By the bacterial cultures of Streptococcus thermophiles and Lactobacillus bulgaricus milk is fermented and yogurt is produced. Through these cultures lactose fermentation takes place to generate lactic acid and with the increase in lactic acid the decrease in ph level of milk helps the milk to clot.

The lactic acid gives the milk a gel based texture that is known as yogurt. These two cultures are mainly used to produce yogurt.  

option a is correct i.e.

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.