Answers to Flammable Hydrogen-Air Mixtures Question
Answer b) is correct.
A spark you can barely feel, e.g., from a static discharge, already has about 50 times as much energy as needed to ignite a mixture of hydrogen and air (21 % oxygen). A typical spark that you experience has over 1000 times the energy required for ignition of the mixture. At higher oxygen concentrations, the mixture is even more easily ignited.
For more information on electrostatic discharges view the previous Chemical Process Safety – Test Your Knowledge