Step 1: Link acidity to phenoxide stability. A phenol is more acidic when the phenoxide ion left behind after losing a proton is more stable. Electron donating groups push electron density into the ring, destabilise that negative ion, and lower acidity. Step 2: Analyse compound I (phenol). Plain phenol has no donating substituent, so its phenoxide is the least destabilised among the three, making it the most acidic. Step 3: Analyse compound II (p-cresol). The methyl group donates electrons by a weak $+I$ effect, slightly destabilising the phenoxide and making it less acidic than phenol. Step 4: Analyse compound III (p-methoxyphenol). The methoxy group donates electrons strongly by resonance ($+M$), which destabilises the phenoxide the most and makes this the least acidic. Step 5: Rank the three. Combining these, acidity follows $I \gt II \gt III$. Step 6: Choose the option. This ordering is option (3). \[ \boxed{I \gt II \gt III} \]
