In world history, the Great Wall of China is famous as a long defensive structure. The first large unified section of this wall was built as a continuous fortification under Emperor Qin Shi Huang around 220 BCE. Approximately in which year did this major early phase of construction of the Great Wall of China begin?

Introduction / Context:
The Great Wall of China is one of the most iconic monuments in world history. It was not built all at once, but over many centuries by different Chinese states and dynasties. However, a major turning point came when Emperor Qin Shi Huang unified several existing frontier walls into a long, continuous defensive system. This large scale construction is usually dated to around 220 BCE and is often treated as the first unified Great Wall. This question checks your ability to recall the approximate historical period of this important construction phase.

Given Data / Assumptions:

• The structure in question is the Great Wall of China.
• The focus is on the first large unified section built under Emperor Qin Shi Huang.
• The time is asked in terms of an approximate year in BCE.
• We assume standard historical chronology used in general knowledge books.

Concept / Approach:
To answer this type of world history question, you need to connect three key ideas. First, remember that Qin Shi Huang, the first emperor of a unified China, ruled in the third century BCE. Second, recall that he ordered the connection and strengthening of earlier state walls to protect the northern frontiers. Third, know that most standard references place this major construction phase around 220 BCE. Since the options are given as approximate dates, we choose the year that best matches the accepted historical estimate.

Step-by-Step Solution:
Step 1: Recall that Emperor Qin Shi Huang unified China and founded the Qin dynasty in the third century BCE.Step 2: Historical sources state that shortly after unification he ordered the linking of existing border walls and the building of new sections to form a long defensive wall.Step 3: This major construction activity is usually dated to around 220 BCE, which lies in the early years of the Qin dynasty.Step 4: Compare the options 220 BCE, 120 BCE, 20 BCE, and 320 BCE with the known historical period of Qin Shi Huang.Step 5: 220 BCE fits correctly with Qin Shi Huang, while the other dates fall in much later or earlier centuries that are not associated with his rule.

Verification / Alternative check:
You can cross check by remembering that the Qin dynasty itself lasted roughly from 221 BCE to 206 BCE. Any major imperial project ordered by Qin Shi Huang must therefore fall within this time span. Among the given options, only 220 BCE lies inside the actual years of his reign and aligns with the commonly cited date for the first large unified Great Wall. The other dates would place the construction either long before or long after the Qin dynasty, which is historically inaccurate for this specific phase.

Why Other Options Are Wrong:
120 BCE is much later and relates more to the Han dynasty, not the first unified construction ordered by Qin Shi Huang.
20 BCE is even later and has no direct connection with the earliest Great Wall project under Qin rule.
320 BCE is earlier than the unification of China under Qin Shi Huang and therefore cannot represent his large scale imperial wall building campaign.

Common Pitfalls:
A common mistake is to assume that the Great Wall was built at one single time in history and to confuse later rebuilding, such as during the Ming dynasty, with the earliest large scale construction. Another error is to remember only the century and then choose an option that belongs to the wrong part of that century. Students may also mix up BCE and CE and accidentally choose a date that is chronologically impossible for Qin Shi Huang. Always link the date to the emperor and dynasty mentioned in the question rather than guessing purely from the number.

Final Answer:
The major early unified construction of the Great Wall of China under Emperor Qin Shi Huang began around the year 220 BCE.