The Answer Explained:

Definition of a Ridge Canal:
A ridge canal is a type of canal that is constructed along the ridge or crest of a hill or mountain. It is designed to divert water from one side of the ridge to the other, allowing for efficient irrigation and drainage.

Explanation of the given options:
a) Watershed canal: A watershed canal refers to a canal that follows the natural contours of a watershed, which is the area of land where all precipitation drains into a common outlet, such as a river or lake. This option does not accurately describe a ridge canal, as it focuses on the entire watershed rather than the specific ridge.
b) Contour canal: A contour canal is a canal that follows the contour lines of the land. It is designed to maintain a constant elevation and prevent water from flowing too quickly down slopes. While a ridge canal may have some similarities to a contour canal in terms of following the land's contour, the two terms are not synonymous.
c) Side slope canal: A side slope canal is a canal that is constructed along the side slope of a hill or mountain. It is designed to prevent erosion and improve water flow. While a ridge canal may be located on a side slope, this term does not accurately describe the specific nature of a ridge canal.
d) Across the contours: This option does not accurately describe a ridge canal. While a ridge canal may cross over the contours of the land, it does so specifically along the ridge or crest of a hill or mountain.

Conclusion:
From the given options, the correct answer is option 'A' - Watershed canal.