The correct answer is C) hydrogen gas is being continuously liberated. During the charging of lead-acid storage batteries, hydrogen gas is generated as a byproduct of the electrochemical reaction. This hydrogen gas is highly flammable and can pose a serious fire and explosion hazard if not properly ventilated. Caution must be exercised to ensure adequate ventilation and to avoid any potential ignition sources, as the buildup of hydrogen gas can create a dangerous situation. The other answer choices are incorrect because: A) the acid does not become weaker during charging, B) both plates are not changing to lead sulfate, and D) lead peroxide in the negative plate is not poisonous, but the hydrogen gas production is the primary safety concern.