Primary alcohols are prepared by the reduction of carboxylic acids. However, lithium aluminium hydride (LiAlH4), which is a strong reducing agent, is not used in this reaction. The correct answer is option 'A', which states that LiAlH4 is not used because it is an expensive reagent.

Explanation:
1. Reducing Agents:
- Reduction reactions involve the addition of hydrogen or the removal of oxygen from a compound. In the case of carboxylic acids, the oxygen atom of the carbonyl group needs to be reduced to convert it into an alcohol.
- Lithium aluminium hydride (LiAlH4) is a commonly used reducing agent that can reduce carboxylic acids to primary alcohols. It is a strong reducing agent that donates hydride ions (H-) in the reaction.

2. Limitations of Lithium Aluminium Hydride:
- Expensive: Lithium aluminium hydride is a relatively expensive reagent compared to other reducing agents. It is not economical to use it for the reduction of carboxylic acids to primary alcohols.
- Reactivity: LiAlH4 is a very strong reducing agent and can also reduce other functional groups present in the molecule, such as ketones, aldehydes, and esters. This high reactivity can lead to unwanted side reactions and lower yields of the desired primary alcohol.
- Safety: Handling and working with LiAlH4 require special precautions due to its reactivity. It is highly flammable and reacts violently with water, acids, and oxidizing agents. Therefore, alternative reducing agents that are safer to handle are preferred.

3. Alternative Reducing Agents:
- There are milder and more economical reducing agents that are commonly used for the reduction of carboxylic acids to primary alcohols. Some examples include:
- Sodium borohydride (NaBH4): It is a mild reducing agent that is safer to handle and more selective towards reducing carboxylic acids.
- Catalytic hydrogenation: This method involves the use of a metal catalyst (such as platinum, palladium, or nickel) and hydrogen gas to selectively reduce carboxylic acids to primary alcohols.

In conclusion, the use of lithium aluminium hydride is avoided in the reduction of carboxylic acids to primary alcohols due to its high cost, reactivity, and safety concerns. Alternative reducing agents, such as sodium borohydride or catalytic hydrogenation, are preferred for these reactions.