Step 1: Understanding the concept of Akhand Bharat
- The idea of Akhanda Bharat means "Undivided India," which envisions India, Pakistan, and Bangladesh as one united cultural and political entity.
- It is rooted in the ideology of cultural nationalism and the belief in the civilizational unity of the Indian subcontinent.
Step 2: Role of Bharatiya Janasangh
- The Bharatiya Janasangh, founded in 1951 by Syama Prasad Mukherjee, was the political arm of the Rashtriya Swayamsevak Sangh (RSS).
- It strongly advocated the concept of Akhand Bharat, promoting the idea of eventual reunification of India and Pakistan.
Step 3: Analyzing the options
- (A) Socialist Party: Focused on socialist principles and reforms, not on Akhand Bharat.
- (B) Bharatiya Janasangh: Correct, this party strongly emphasized Akhand Bharat.
- (C) Indian National Congress: Advocated secular democracy and accepted partition, not Akhand Bharat.
- (D) None of these: Incorrect, since option (B) is correct.
Step 4: Conclusion
Thus, the Bharatiya Janasangh was the political party that promoted the idea of uniting India and Pakistan into Akhand Bharat.
\[
\boxed{\text{Correct Answer: (B) Bharatiya Janasangh}}
\]