The mass of the block is
(0.08 m³) x (7,840 kg/m³) = 627.2 kg.
The force of gravity on the block is
(mass) x (gravity)
= (627.2 kg) x (9.8 m/s²)
= 6,146.6 newtons .
As long as the block is on Earth, that's its weight. In vacuum, in air,
in water, in sugar, in bed, in trouble, or in molasses ... that's the force
of gravity attracting it toward the center of the Earth.
If it happens to be in water, then there's another force acting on it ...
the upward force of buoyancy, equal to the weight of the water it
displaced.
The block displaced 0.08 m³ of water, so the buoyant force is the weight
of 0.08 m³ of water.
The density of water is 1,000 kg/m³, so the mass of the displaced
water is
(0.08 m³) x (1,000 kg/m³) = 80 kg .
and its weight is (80 kg) x (9.8 m/s²) = 784 newtons.
That's the buoyant force on the block . . . 784 newtons.
It acts upward on the block, opposite to the force of gravity.
So, if there's a bathroom scale on the bottom of the pool,
and the block is resting on it, the scale will read
(actual weight) minus (buoyant force)
= (6,146.6 newtons) - (784 newtons) = 5,362.6 newtons .
The real weight of the block is 6146.6 newtons.
That's choice-D.
The apparent weight of the block in water is 5,362.6 newtons.
That's choice-A.
The question asked for the force of gravity on the block, and then
in parentheses it says (weight). Both of these are 6146.6 newtons.
If you answer 'D', and your answer is marked wrong and somebody
tries to tell you that the correct answer is 'A', then I want you to either
show them what I have written here, or else tell them to come see me,
and I will straighten them out.
