The original name of 'Mahabharata' is 'Jai Samhita'.

Explanation:

Introduction to Mahabharata:
Mahabharata is one of the two major Sanskrit epics of ancient India, the other being the Ramayana. It is an epic narrative of the Kurukshetra War and the fates of the Kaurava and Pandava princes. The Mahabharata is traditionally ascribed to sage Vyasa, who is also a major character in the epic. It is one of the longest epic poems ever written, consisting of over 200,000 verses divided into 18 books.

Explanation of the Options:
a) Katha Saritsagar: Katha Saritsagar is a collection of Indian legends, fairy tales, and folk tales retold in Sanskrit by Somadeva Bhatta in the 11th century. It is not the original name of Mahabharata.

b) Jai Samhita: Jai Samhita is the original name of Mahabharata. However, it is important to note that the term 'Jai Samhita' is not commonly used to refer to the epic. The more widely known and accepted name is 'Mahabharata.'

c) Vrahat Katha: Vrahat Katha is a collection of ancient Indian stories written by Gunadhya in Paisachi language. It is not related to Mahabharata.

d) Rajatarangini: Rajatarangini is a historical chronicle of the kings of Kashmir written by Kalhana in the 12th century. It is not related to Mahabharata.

Conclusion:
In conclusion, the original name of Mahabharata is 'Jai Samhita.' However, it is important to note that this name is not commonly used, and the epic is widely known as 'Mahabharata.'