Identify the species

Orthoboric acid is boric acid, $\mathrm{H_3BO_3}$, commonly written as $\mathrm{B(OH)_3}$.

Key concept: why is boric acid acidic?

Boric acid is not a protonic acid in water in the usual sense. It behaves as a weak acid by accepting $\mathrm{OH^-}$ from water:

Thus, its acidity is due to Lewis acid behaviour, not due to self-ionisation.

Check each option

Option A: "it behaves as a weak acid in water due to self ionisation"

This is incorrect.

Boric acid does not show acidity because of self-ionisation like a typical Brønsted acid. Its acidity arises because it accepts $\mathrm{OH^-}$ from water and generates $\mathrm{H_3O^+}$ indirectly.

So, A is false.

Option B: "acidity of its aqueous solution increases upon addition of ethylene glycol"

This is correct.

Ethylene glycol (like glycerol and other polyhydroxy compounds) forms a complex with boric acid/borate ion. This stabilizes the conjugate base and increases acidity.

Hence, B is true.

Option C: "it has a three dimensional structure due to hydrogen bonding"

This is incorrect.

In the solid state, orthoboric acid has a layered planar structure. Within a layer, molecules are linked by hydrogen bonding, but it is not a three-dimensional network.

So, C is false.

Option D: "it is a weak electrolyte in water"

This is correct.

Since boric acid ionizes only slightly in water, its aqueous solution conducts electricity weakly.

Therefore, D is true.

Final derived answer

The correct statements are:

Comparison with stored correct answer

Stored correct answer: $B, D$

My derived answer matches the stored answer.