Exhaust gas recirculation (EGR) is a technique used in internal combustion engines to reduce nitrogen oxide (NOx) emissions. It works by recirculating a portion of the engine's exhaust gas back into the intake manifold, where it mixes with the fresh air-fuel mixture before entering the combustion chamber.

Decreasing thermal efficiency:
- One disadvantage of EGR is that it can decrease the thermal efficiency of the engine. This is because when the exhaust gas is recirculated, it displaces a portion of the fresh air-fuel mixture in the combustion chamber, leading to a lower combustion efficiency.
- The presence of exhaust gas reduces the oxygen concentration in the combustion chamber, which affects the combustion process and can result in incomplete fuel combustion.
- Incomplete combustion leads to the production of less useful work and increases the fuel consumption, thereby reducing the thermal efficiency of the engine.

Increasing HC emission:
- Another disadvantage of EGR is that it can increase the emission of hydrocarbon (HC) pollutants. HC emissions are a result of incomplete combustion, and as mentioned earlier, the presence of exhaust gas in the combustion chamber can lead to incomplete combustion.
- The recirculated exhaust gas contains unburned hydrocarbons, which can further contribute to the HC emissions.

Both a and b:
- Therefore, the correct answer is option 'c' - EGR has the disadvantages of decreasing thermal efficiency and increasing HC emissions.
- The decrease in thermal efficiency and increase in HC emissions are two significant drawbacks of using EGR in internal combustion engines.
- These disadvantages need to be carefully considered when implementing EGR systems, and engineers should strive to find a balance between reducing NOx emissions and maintaining optimal engine performance and efficiency.

Increasing aldehydes:
- The option 'd' - increasing aldehydes - is not a correct answer because EGR does not have any direct influence on the production of aldehydes.
- Aldehydes are organic compounds that can be formed during the combustion process, but their formation is not specifically attributed to the use of EGR. Other factors such as fuel composition, combustion conditions, and catalyst efficiency can contribute to the production of aldehydes.

In conclusion, while exhaust gas recirculation has the advantages of reducing NOx emissions, it also has the disadvantages of decreasing thermal efficiency and increasing HC emissions. These drawbacks should be taken into consideration when implementing EGR systems in internal combustion engines.