Concept:
Historical chronology questions are solved by identifying the approximate historical periods of kingdoms, dynasties, and cultural developments.
Step 1: Finding the period of Vedas and Upanishads.
The Vedas and Upanishads belong to ancient Indian civilization and were composed roughly between:
\[
1500 \text{ BCE to } 500 \text{ BCE}
\]
This is the earliest event among the given options.
Thus:
\[
C \text{ comes first}
\]
Step 2: Finding the period of Kushan Dynasty.
The Kushan Dynasty flourished approximately during:
\[
1^{st} \text{ to } 3^{rd} \text{ century CE}
\]
It came much later than the Vedic period.
Hence:
\[
D \text{ comes after } C
\]
Step 3: Finding the period of Khmer Kingdom.
The Khmer Kingdom in Cambodia became prominent around:
\[
9^{th} \text{ century CE onwards}
\]
It is famous for:
• Angkor civilization,
• Temple architecture,
• Hindu-Buddhist cultural influence.
Therefore:
\[
A \text{ comes after } D
\]
Step 4: Finding the period of Mughal Empire.
The Mughal Empire in India began in:
\[
1526 \text{ CE}
\]
It belongs to the medieval period and is the latest among the given options.
Thus:
\[
B \text{ comes last}
\]
Step 5: Final chronological arrangement.
Therefore, the correct sequence is:
\[
\text{Writing of Vedas and Upanishads} \rightarrow \text{Kushan Dynasty} \rightarrow \text{Khmer Kingdom} \rightarrow \text{Mughal Empire}
\]
That is:
\[
\boxed{C, D, A, B}
\]