Assertion: Before its disintegration in 1991, the USSR consisted of 15 Soviet republics.

Reason: Estonia becomes the first of the 15 Soviet republics to declare its independence from the Soviet Union in 1990.

The Assertion is correct but the Reason is incorrect.

The correct answer is Option 4 - The Assertion is correct but the Reason is incorrect.

Assertion: Before its disintegration in 1991, the USSR consisted of 15 Soviet republics.
Reason: Estonia becomes the first of the 15 Soviet republics to declare its independence from the Soviet Union in 1990.

CORRECTION in Reason: Lithuania becomes the first of the 15 Soviet republics to declare its independence from the Soviet Union in 1990.

The independence movement began in Lithuania in 1988, and later spread to Estonia and Latvia.
Lithuania became the first of the 15 Soviet republics to declare its independence in March 1990.

Lithuania was the first republic to declare independence from the Soviet Union by the Act of March 11, 1990 with its Baltic neighbours and the Southern Caucasus republic of Georgia joining it in a course of two months.