Step 1: Understanding the Concept:
The question asks to identify which instrument from the given list is not a traditional part of the ensemble for the Bihu dance of Assam.
Step 2: Detailed Explanation:
The traditional musical ensemble for Bihu is quite specific and has a distinct rustic sound. The core instruments include:
1. Dhol: A large, double-sided barrel drum played with a stick and a hand. It provides the main rhythm and is indispensable to Bihu.
4. Gagana: A small instrument made of bamboo, played by holding it between the teeth and plucking its reed. It is a type of jaw harp and is a characteristic Bihu instrument.
Other traditional instruments include the \textit{Pepa} (a hornpipe made from a buffalo horn), \textit{Toka} (a bamboo clapper), and \textit{Xutuli} (a clay whistle).
2. Takka: While less common than the Dhol, 'Takka' or similar percussion instruments can be found in various folk traditions. However, the most definitively non-traditional instrument on the list is the Violin.
3. Violin: The violin is a Western classical string instrument. While modern and fusion Bihu performances may sometimes incorporate it for melodic accompaniment, it is not a part of the traditional Bihu ensemble. The question refers to the established tradition of the dance.
Step 3: Final Answer:
The Violin is not a traditional musical instrument used in Assam's Bihu dance. Therefore, option (3) is the correct answer.