In space crafts, which of the following substances is commonly used to prevent the fuel from freezing at extremely low temperatures?

Introduction / Context:
Spacecraft operate in extremely cold environments, where many liquids, including fuels and coolants, are at risk of freezing. Engineers therefore use special additives called antifreezes to lower the freezing point of these liquids. This question asks which of the listed substances is used for preventing fuel from freezing in space crafts. Understanding the basic role of glycols as antifreeze agents reinforces fundamental ideas about colligative properties and practical applications of chemistry in space technology.

Given Data / Assumptions:
- The setting is a space craft, where fuel can be exposed to very low temperatures.
- The design requirement is to prevent the fuel from freezing, so an antifreeze or freezing point depressant is needed.
- The options given are benzene, glycol, acetylene, and ester, all of which are organic compounds.
- We assume standard knowledge of which of these substances is widely used as an antifreeze in engineering systems.

Concept / Approach:
Antifreeze agents commonly used in engines and industrial cooling systems are glycols, such as ethylene glycol or propylene glycol. These compounds mix with water or other liquids and significantly lower the freezing point due to colligative effects, preventing the mixture from solidifying at low temperatures. Benzene is a volatile aromatic hydrocarbon, acetylene is a flammable gas used as a fuel in welding, and ester is a general term for a class of compounds with many uses, but none of these are the standard antifreeze agents used in such systems. Therefore, glycol is the best answer among the options.

Step-by-Step Solution:
Step 1: Recall that antifreeze formulations used in automotive radiators and many cooling systems are based on ethylene glycol or propylene glycol. Step 2: Recognise that the same basic chemistry, lowering of freezing point by mixing glycols with fuels or coolants, can be used in aerospace applications to protect against freezing. Step 3: Examine benzene, which is a liquid hydrocarbon but not a standard antifreeze additive and possesses serious health hazards. Step 4: Examine acetylene, which is a gas used for high temperature welding and is not suitable as a liquid antifreeze agent. Step 5: Examine the generic term ester, which describes a wide group of compounds without specifically indicating antifreeze properties. Step 6: Conclude that glycol is the substance typically used to prevent freezing, making it the correct answer.

Verification / Alternative check:
Reference materials on automotive and aerospace engineering refer repeatedly to glycols as key components in antifreeze and coolant mixtures. Ethylene glycol based fluids are standard in many terrestrial engines, while propylene glycol is often chosen when lower toxicity is desired. Both types act by lowering the freezing point and raising the boiling point of the mixture. No similar widespread use is reported for benzene, acetylene, or generic esters as primary antifreeze additives in fuel systems. This technical background supports glycol as the correct option in an exam context.

Why Other Options Are Wrong:
Benzene: An aromatic solvent and fuel component, but not a common antifreeze additive due to toxicity and volatility.
Acetylene: A gaseous hydrocarbon used mainly for welding and cutting, not for freezing point depression in fuels.
Ester: A broad class of compounds with many uses, including fragrances and plastics, but not a standard antifreeze material mentioned in basic science texts for this purpose.

Common Pitfalls:
Students may be tempted to choose benzene or another hydrocarbon simply because these are associated with fuels and organic chemistry. Another mistake is to interpret the word ester as a specific technological material rather than a broad functional group. To avoid these errors, it helps to associate the word glycol directly with antifreeze, as seen in automotive coolant labels and science demonstrations that show freezing point depression using ethylene glycol or propylene glycol.

Final Answer:
In space crafts, substances such as Glycol are used as antifreeze agents to prevent fuel from freezing at very low temperatures.