Concept:
The Dandi March was one of the most significant movements of the Indian freedom struggle led by Mahatma Gandhi against British colonial rule.
Step 1: Examine Assertion A.
Mahatma Gandhi selected:
\[
\text{Salt}
\]
as a symbol during the Civil Disobedience Movement and Dandi March.
Salt was chosen because it was a daily necessity used by every individual, rich or poor.
Thus, Assertion A is correct.
Step 2: Examine Reason R.
The British government had imposed:
• Heavy taxes on salt
• Monopoly over salt production and sale
This policy was highly unpopular among Indians because salt was an essential commodity.
Hence, Reason R is also correct.
Step 3: Check whether Reason explains Assertion.
Gandhi strategically chose salt because the British monopoly over salt affected common people and generated widespread dissatisfaction.
Therefore, Reason R correctly explains Assertion A.
Step 4: Conclusion.
Hence, the correct answer is:
\[
\boxed{\text{Both A and R are correct and R is the correct explanation of A}}
\]