Question

if the earth shrinks to 50% of its real radius its mass remaining the same,the weight of a body on the earth will increase or decrease?​

Answer 1

decrease,due to the decrease in gravity

Answer 2

The weight of a body on earth will increase since the density of earth decreases.
You can see this on using Newton's Gravitational Formulae:-
The force of earth is directly proportional to the product of their masses, which According to the question will remain constant. But is inversely proportional to the square of the radius of earth, which According to the question will decrease and hence the overall force on the body will go up.
Thus, the weight of the body will increase.