Assertion: In December 1961, the Government of India sent the army to Goa which liberated territories of Goa, Diu and Daman from Portuguese.

Reason: Portugal had refused to withdraw from the territories of Goa, Diu and Daman which were under its colonial rule since the sixteenth century.

Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion

Goa's liberation: Although the British empire in India came to an end in 1947, Portugal refused to withdraw from the territories of Goa, Diu and Daman which were under its colonial rule since the sixteenth century. During their long rule, the Portuguese suppressed the people of Goa, denied them civil rights, and carried out forced religious conversions. After India’s Independence, the Indian government tried very patiently to persuade the Portuguese government to withdraw. There was also a strong popular movement within Goa for freedom. They were strengthened by socialist satyagrahis from Maharashtra. Finally, in December 1961, the Government of India sent the army which liberated these territories after barely two days of action. Goa, Diu and Daman became Union Territory.