The correct answer is B) Odor. The primary way to detect escaping gas is by smell or odor. Natural gas and other flammable gases typically have an added odorant, such as mercaptan, to give them a distinct sulfurous or rotten egg-like smell. This odor alerts personnel to the presence of a gas leak, allowing them to take appropriate safety measures. The other answer choices are not the primary means of detecting escaping gas. Running a hand along a pipe (A) may help identify a leak by feel, but would not reliably detect the presence of gas. Increasing the line pressure (C) could potentially exacerbate the leak. A red flame (D) indicates a combustible gas mixture, but would not be the initial detection method.