a) The combustion of an organic compound in the presence of excess oxygen produced a gas , which makes clear lime water turbid. What element can be identified? 

the COMPOUND is carbon dioxide ... CO₂

the element is carbon dioxide.

To identify the element responsible for making clear lime water turbid, we need to follow these steps:

1. Understand the chemical reaction: When an organic compound undergoes combustion in the presence of excess oxygen, it typically produces carbon dioxide (CO2) gas and water (H2O).

2. Observe the reaction with lime water: Lime water is a solution of calcium hydroxide (Ca(OH)2). It is clear in its original state and reacts with carbon dioxide gas (CO2) to form a white precipitate of calcium carbonate (CaCO3), which makes the lime water turbid, or cloudy.

Based on these steps, we can conclude that the element responsible for making clear lime water turbid in this scenario is carbon.