Explanation

(a)
Orthophosphoric acid(H_{3}PO_{4}) is a tribasic acid. Hence, its structure can be represented as $O \longleftarrow P(OH)_{3}$.
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($lp + \sigma bp = 1 + 3 = 4)$
Hence hybridization of p in H_{3}PO_{4} is sp^{3} and thus it is tetrahedral in shape.