The whole outcome is decided by two facts: the \(-OH\) group steers nitro groups to the 2, 4 and 6 carbons, and the strength of the acid decides how many nitro groups get in.
Dilute nitric acid route: A weak nitrating mixture can only manage a single substitution. Since the free ortho and para sites are both open, phenol gives two mononitro products side by side. One is 2-nitrophenol, where the nitro group sits next to the hydroxyl (this isomer is steam volatile because of intramolecular hydrogen bonding). The other is 4-nitrophenol, with the nitro group directly across the ring (higher boiling, held by intermolecular hydrogen bonds). These are (A) and (B).
Concentrated nitric acid route: A strong nitrating medium keeps substituting until all three activated positions, carbons 2, 4 and 6, carry a nitro group. The final compound is 2,4,6-trinitrophenol, the pale yellow explosive better known as picric acid. This is (C).
Structures in words: (A) hydroxyl on C1 and nitro on C2; (B) hydroxyl on C1 and nitro on C4; (C) hydroxyl on C1 with nitro groups on C2, C4 and C6.
\(\boxed{A = o\text{-nitrophenol},\ B = p\text{-nitrophenol},\ C = \text{picric acid}}\)