Step 1: Identify the regional language of Bihar.
Bhojpuri is the most widely spoken regional language in Bihar, along with Hindi.
Bundeli: A language spoken in parts of Madhya Pradesh, not Bihar.
Bihari: A group of languages, but Bhojpuri is the prominent one in Bihar.
Khasi: Spoken in Meghalaya, not Bihar.
Step 2: Hence, the correct answer is (1) Bhojpuri, the prominent regional language of Bihar.