Which of the following princely states signed the 'instrument of accession' after the independence of India?

All of the above.

Prior to 15 August 1947, the peaceful negotiations between the Indian government and the rulers of most states resulted in the inclusion of their territories, contiguous to the new boundaries of India, into the Indian Union. The rulers willingly signed the "Instrument of Accession," indicating their state's agreement to become a part of the Union of India.

Each of these princely states signed the 'instrument of accession' after independence:

Junagadh:
A plebiscite was conducted, and most of Junagadh's population voted to join India.
The Government of India accepted the accession of Junagadh & Junagadh officially became a part of India on February 20, 1948.

Kashmir:
In October 1947, armed tribesmen from Pakistan invaded Kashmir with the aim to annex Kashmir to Pakistan.
Faced with the invasion and deteriorating security situation, Maharaja Hari Singh signed the Instrument of Accession on October 26, 1947, acceding to India.
Following the accession, India sent troops to Kashmir to repel the invading tribesmen.

Hyderabad:
The Indian government launched "Operation Polo" in September 1948 to bring Hyderabad under Indian control.
The Indian military swiftly defeated the Hyderabad State Forces, and Nizam's administration surrendered.
In September 1948, the Nizam signed the Instrument of Accession, formally integrating Hyderabad into the Indian Union.