Metaphosphoric acid is:

$(HPO_3)_3$

The correct answer is Option (3) → $(HPO_3)_3$.

Metaphosphoric acid refers to a polymeric form of phosphoric acid where the general formula is \( (HPO_3)_n \), where \( n \) is typically 3 or higher.

For the given options, \( (HPO_3)_3 \) is the correct representation of cyclic trimetaphosphoric acid, which is the trimer of metaphosphoric acid

Breakdown of the options:

H₃PO₃: This is phosphorous acid, not metaphosphoric acid.

H₄P₂O₇: This is pyrophosphoric acid.

H₄P₂O₆: This is hypophosphoric acid.

Thus, (HPO₃)₃ is the correct formula for metaphosphoric acid.