The traditional wooden cantilever bridge of Singshore in West Sikkim is known for being:

The correct answer is: c) A historical monument.

The Singshore Bridge is a traditional wooden cantilever bridge in West Sikkim, India. It is one of the oldest bridges in Sikkim and is a popular tourist destination. The bridge is made of wood and bamboo and is supported by stone piers. It is about 100 meters long and 3 meters wide. The bridge was built in the 18th century and is a fine example of traditional Sikkimese architecture. It is a popular spot for photography and picnicking.

The other options are incorrect because:

• The Singshore Bridge is not the highest bridge in Sikkim. The highest bridge in Sikkim is the Rangpo Bridge, which is a suspension bridge that crosses the Teesta River.
• The Singshore Bridge is not a sacred site. There are no religious or spiritual significance associated with the bridge.
• The Singshore Bridge is not an ancient trade route. The bridge was built in the 18th century, which is relatively recent in the history of trade routes.