During the reign of which Chinese dynasty was the first unified Great Wall of China substantially constructed to defend the northern frontier?

Introduction / Context:
The Great Wall of China is one of the most famous structures in world history and a symbol of Chinese civilisation. Although various states and dynasties built walls at different times, there was a key moment when many regional walls were linked and strengthened to form a more unified defensive system against northern nomadic groups. This question asks you to identify the dynasty under whose rule that early unified Great Wall was substantially constructed, a popular fact in general history exams.

Given Data / Assumptions:

• The structure discussed is the Great Wall of China in its early unified form.
• The options list the Song, Tang, Han and Qin (Chin) dynasties.
• You must determine which dynasty is associated with the first large scale unification of northern defensive walls.
• We assume basic knowledge of the order of major Chinese dynasties and their achievements.

Concept / Approach:
The Qin dynasty, sometimes spelled Chin, was the first imperial dynasty of a unified China under Emperor Qin Shi Huang in the third century BCE. After conquering rival states, the Qin rulers ordered workers to connect and extend existing northern walls to create a long continuous barrier against northern nomadic groups. This project is often identified as the early form of the Great Wall. Later dynasties, including the Han and much later the Ming, repaired and rebuilt sections, but the first major unification is usually credited to the Qin. Therefore, the correct approach is to associate the first unified Great Wall with the Qin dynasty.

Step-by-Step Solution:
Step 1: Recall that the Qin dynasty under Qin Shi Huang unified China around the third century BCE. Step 2: Remember that after unification, the Qin government sought to secure northern borders against nomadic raids. Step 3: Note that the Qin ordered the linking and strengthening of existing regional walls, creating a long defensive structure. Step 4: Recognise that later dynasties such as the Han and Ming continued to build and rebuild sections, but the first large unified project is tied to the Qin. Step 5: Select Qin (Chin) dynasty as the correct answer from the options provided.

Verification / Alternative Check:
History texts about China often summarise the Great Wall by noting that walls existed earlier in separate states, but Emperor Qin Shi Huang ordered them joined into a more continuous barrier. They also point out that the present day wall segments familiar to tourists are largely from the later Ming dynasty, but that the concept and early structure were Qin projects. The Song and Tang dynasties came much later and are associated with other achievements, not with the original unification of the wall. The Han dynasty followed Qin and extended some defences, yet most exam questions on the first unified Great Wall point to Qin. These points confirm the correct choice.

Why Other Options Are Wrong:
The Song dynasty ruled in a later period and struggled with northern invasions itself, but it did not initiate the earliest unified Great Wall. The Tang dynasty presided over a powerful empire and cultural flowering but is not primarily associated with the first construction of the wall. The Han dynasty did extend and maintain parts of the defence system, yet the credit for the initial large scale unification of walls goes to the Qin. Therefore, these options either refer to later work or focus on dynasties known for other accomplishments.

Common Pitfalls:
Students sometimes confuse the dynasty that built the first wall with the one that built the best preserved sections seen today, which are mostly from the Ming period. Another pitfall is mixing up Han and Qin because both appear early in Chinese history. To avoid such mistakes, remember the sequence Qin first, then Han, and link Qin with unification: unification of China and unification of defensive walls. If you keep that association in mind, you will be more likely to choose the correct answer in questions about the Great Wall.

Final Answer:
The Great Wall of China in its first unified form was substantially constructed under the Qin (Chin) dynasty, so that option is correct.