The acceleration due to gravity is 9.8 m/s². When the body is thrown upward, its initial velocity is positive. As the body moves upward, its velocity decreases due to the force of gravity. When the velocity of the body becomes zero, it reaches its maximum height.

Let h be the maximum height attained by the body.

Let u be the initial velocity of the body.

Let t be the time taken by the body to reach its maximum height.

We know that:

v = u - gt

where v is the final velocity of the body.

When the body reaches its maximum height, its final velocity is zero. Therefore,

0 = u - gt

or,

t = u/g

We also know that:

h = (u² - v²)/2g

Substituting v = 0 in the above equation, we get:

h = u²/2g

Now, we are given that the velocity of the body is reduced into half in one second. Therefore,

(1/2)u = u - (9.8 m/s² × 1 s)

or,

u = 19.6 m/s

Substituting the value of u in the equation for h, we get:

h = (19.6 m/s)²/2(9.8 m/s²)

h = 19.6 m

Therefore, the maximum height attained by the body is 19.6 meters.