The correct answer is C. The men entering the manhole should be advised to smoke in the sewer.
Smoking in a sewer is a dangerous practice that can lead to serious health problems. The fumes from cigarettes can contain harmful chemicals, such as carbon monoxide, that can be fatal if inhaled in high concentrations. Additionally, the smoke can create a fire hazard in the sewer, which could put workers at risk.
For these reasons, it is important to avoid smoking in sewers. Workers who must enter sewers should be advised to refrain from smoking, and employers should provide safe and healthy working conditions for their employees.
The other options are all correct. Option A states that the particular manhole and one manhole on upstream and one manhole on downstream should remain open for 30 minutes. This is important to ensure that there is adequate ventilation in the sewer and that workers are not exposed to harmful gases. Option B states that proper tests for the presence of poisonous gases must be carried out. This is essential to ensure the safety of workers before they enter the sewer. Option D states that warning signals should be erected. This is important to alert other workers and the public to the presence of workers in the sewer.