Refer to figure.
Spark plug appearance provides direct diagnostic information about combustion conditions and mechanical integrity of the engine. Different deposit types have characteristic appearances that allow trained maintenance personnel to identify the underlying cause. 

Black oily wet deposits on the firing end of a spark plug are specifically associated with oil contamination of the combustion chamber. When piston rings are worn, damaged, or have lost their sealing effectiveness, engine lubrication oil from the crankcase can pass the ring seal and enter the combustion chamber. This oil is then partially combusted during the power stroke but leaves a characteristic wet, dark, oily carbon residue on the spark plug, cylinder walls, and combustion chamber surfaces. 

The wet and oily nature of the deposit distinguishes it clearly from the dry powdery black carbon associated with an over-rich fuel mixture. Oil-fouled spark plugs may also cause misfiring due to the conductive oil film bridging the electrode gap and shorting the spark. Excessive oil consumption, blue smoke in the exhaust, and oil-fouled spark plugs together are classic diagnostic indicators of worn piston rings or valve stem seals.

That the mixture is too lean → INCORRECT. A lean mixture burns hotter and tends to produce white, grey, or blistered deposits on spark plugs, sometimes accompanied by electrode erosion or melting in extreme cases. A lean mixture does not produce black oily deposits. The black oily appearance is specifically linked to oil contamination, not to lean mixture conditions.

The presence of water in the fuel → INCORRECT. Water contamination in fuel does not produce black oily carbon deposits on spark plugs. Water in fuel typically causes misfiring, power loss, and rough running, and any residual evidence would manifest as corrosion, pitting, or white mineral deposits rather than black oily carbon. The black oily appearance is inconsistent with water contamination.

That oil is slipping past the piston rings and is being burnt in the combustion chamber → CORRECT. Black wet oily deposits on the spark plug firing end are the classic signature of oil bypass past worn or damaged piston rings. The oil enters the combustion chamber, undergoes partial combustion, and deposits a wet carbonaceous oily residue on the plug. This finding, combined with high oil consumption and blue exhaust smoke, confirms oil ring bypass as the cause.

That the engine has been operated with too rich a mixture → INCORRECT. A rich mixture produces DRY black powdery carbon deposits, not wet oily ones. The dry powdery texture results from unburned fuel carbon settling on the plug surfaces. The wet and oily texture of the deposits in this question is the distinguishing feature that identifies oil contamination rather than rich mixture as the cause.