While India is not unique in experiencing one-party dominance, many other countries worldwide have encountered similar situations. However, a critical distinction sets the Indian experience apart. In numerous other cases, the dominance of a single party was established through compromising democratic principles.

Which of the following country was once a one-party dominant state?

All of the above.

India is not the only country with one-party dominance; many other nations have experienced it.
The major difference is that in other cases, the preponderance of one party was attained by compromising democracy.
Countries like China, Cuba, and Syria have constitutions that permit only a single ruling party.
Other countries, including Myanmar, Belarus, Egypt, and Eritrea, effectively function as one-party states due to legal and military measures.
Mexico, South Korea, and Taiwan were also effectively one-party dominant states until a few years ago