At birth, the average brain volume is 20 cubic inches (341 cubic centimetres), and at 90 days, it is 34 cubic inches (558 cubic centimetres). In other words, babies brain developed from 33 percent to 55 percent of the average adult brain size in three months.
During the first few years of life, the majority of the brain's wiring is established. It was only approximately a fifth of its adult size when it was born. However, by the age of two, it has grown to three-quarters of adult size! By the age of five, the brain will have reached adult size and volume.
Thus, the correct option is 2.