The mid-1st millennium BCE is regarded as a turning point in history because,

(a) New Kingdoms and cities developed
(b) Changes in social and economic life in Ganga Valley.
(c) Emergence of thinkers such as Zarathustra, Plato, etc
(d) Temple architecture emerged
(e) Ideas in Upanishad talk about meaning of life & death

Choose the correct answer from the options given below:

(a), (b), (c), (e) only

The correct answer is Option (1) → (a), (b), (c), (e) only

(a) New Kingdoms and cities developed
(b) Changes in social and economic life in Ganga Valley.
(c) Emergence of thinkers such as Zarathustra, Plato, etc
(d) Temple architecture emerged
(e) Ideas in Upanishad talk about the meaning of life & death

Temple architecture emerged is an incorrect point for being one of the turning point of the mid-1st millennium BCE because temple architecture became prominent much later, during the early centuries CE, particularly under Gupta rule. It was not a hallmark of the mid-1st millennium BCE.

Remaining points:

The mid-first millennium BCE is often regarded as a turning point in world history: it saw the emergence of thinkers such as Zarathustra in Iran, Kong Zi in China, Socrates, Plato and Aristotle in Greece, and Mahavira and Gautama Buddha, among many others, in India. They tried to understand the mysteries of existence and the relationship between human beings and the cosmic order. This was also the time when new kingdoms and cities were developing and social and economic life was changing in a variety of ways in the Ganga valley. These thinkers attempted to understand these developments as well.

Many ideas found in the Upanishads (c. sixth century BCE onwards) show that people were curious about the meaning of life, the possibility of life after death and rebirth. Was rebirth due to past actions? Such issues were hotly debated. Thinkers were concerned with understanding and expressing the nature of the ultimate reality. And others, outside the Vedic tradition, asked whether or not there even was a single ultimate reality. People also began speculating on the significance of the sacrificial tradition.