Spalding found that if newborn birds were prevented from hearing their mother's call for five days, they would still recognize her; if prevented from hearing the mother for 10 days, they would never learn to recognize her. That is, he demonstrated the existence of what later was called.

critical periods

Explanation:

Critical period, in developmental biology or psychology, is described as a "maturational stage" in an organism's (animal, human) lifespan during which the organism's nervous system is specifically sensitive to a few environmental stimuli. However, the critical period is the time in which an individual develops language-based cues i.e, around five years to puberty after which it becomes difficult to develop language acquisition and the person is quite unsuccessful in that.

As per the question, the correct answer is critical periods.  

dear friend:

  i hope this letter finds you in soaring spirits and vibrant health. as you had expressed the wish to know about culture, traditions, and unity of india in your previous letter, in the following lines i have tried my best to give you glimpses of our culture, traditions, and unity. you will be amazed to know that our culture is the oldest culture in the world and the birth place of hinduism, buddhism, jainism, and sikhism. you won’t find any other culture as varied and multi-colored, multi-lingual as ours. each region has its own unique culture, traditions, language. there are 22 official languages in india.

  basically, india is a spiritual country with orthodox traditions. for example the people of india believe in traditions set by their wise forefathers. arranged marriages, joint-family system, universal brotherhood, vegetarianism, celebrating festivals such as diwali, holi, eid, etc., are the salient features of our culture and traditions.

  another unique aspect about our country is ‘unity in diversity’. in spite of the multi-lingual, multi-religious, multi-traditional, multi-cultural society, people live peacefully together, respecting one another’s views, cultures, traditions, and differences. i am sure you would love to visit india soon, and have a first-hand experience of the multi-colored culture. give regards to uncle and aunt. do visit india with your family and experience our hospitality.

your friend,