Step-by-step Solution:

1. Understand the question: The question asks to identify a group of metals that commonly occur in nature as sulphide ores. Sulphide ores are mineral compounds where the metal is bonded to sulfur.

2. Analyze the nature of metals forming sulphide ores: Generally, metals with lower reactivity and higher affinity for sulfur (chalcophiles) tend to form sulphide ores. These are typically found in the p-block and d-block of the periodic table. Highly reactive metals like those in Group 1 and Group 2 (alkali and alkaline earth metals) and Aluminium are more electropositive and have a high affinity for oxygen, so they are usually found as oxides, carbonates, or halides.

3. Evaluate each option:

   • A: Ag, Cu and Pb

     • Silver (Ag): The main ore of silver is Argentite or silver glance, which has the chemical formula $Ag_2S$. This is a sulphide ore.
     • Copper (Cu): Copper has several important sulphide ores, including Copper pyrites ($CuFeS_2$), Chalcocite ($Cu_2S$), and Covellite ($CuS$).
     • Lead (Pb): The principal ore of lead is Galena, with the formula $PbS$. This is a sulphide ore.
     • Since all three metals in this option commonly form sulphide ores, this option is a strong candidate.

   • B: Ag, Mg and Pb

     • Silver (Ag): Forms sulphide ore ($Ag_2S$).
     • Magnesium (Mg): Magnesium is a reactive alkaline earth metal (Group 2). It is highly electropositive and is found as carbonate (Magnesite, $MgCO_3$), chloride (Carnallite, $KCl.MgCl_2.6H_2O$), or sulphate (Epsom salt, $MgSO_4.7H_2O$). It does not commonly form sulphide ores.
     • Lead (Pb): Forms sulphide ore ($PbS$).
     • Because Magnesium does not form sulphide ores, this option is incorrect.

   • C: Ag, Cu and Sn

     • Silver (Ag): Forms sulphide ore ($Ag_2S$).
     • Copper (Cu): Forms sulphide ores (e.g., $CuFeS_2$).
     • Tin (Sn): The principal ore of tin is Cassiterite, which is an oxide ore ($SnO_2$). While a sulphide ore of tin (Stannite, $Cu_2FeSnS_4$) exists, it's not as common as its oxide ore.
     • This option is less correct than option A because tin is primarily found as an oxide.

   • D: Al, Cu and Pb

     • Aluminium (Al): Aluminium is a highly reactive metal (Group 13). Its main ore is Bauxite ($Al_2O_3.xH_2O$), which is a hydrated oxide ore. Aluminium does not form sulphide ores due to its high affinity for oxygen.
     • Copper (Cu): Forms sulphide ores.
     • Lead (Pb): Forms sulphide ore.
     • Because Aluminium does not form sulphide ores, this option is incorrect.

4. Conclusion: Based on the analysis, Option A is the only group where all three metals (Ag, Cu, and Pb) are commonly and principally found as sulphide ores. Therefore, it is the correct answer.