What is another name for an acid hydrometer?
[A] Lactometer
[B] Acidometer
[C] Battery hydrometer
[D] Barkometer

Hint: In order to solve this question we need to understand what acid and specific gravity are. Acid is any substance having hydrogen and it is capable of donating a proton to another substance, on the litmus paper it turns from blue to red. Specific gravity is defined as ratio of density of substance to the density of reference object, for example specific gravity of a molecule in crystal is defined as ratio of density of molecule to the density of crystal.

Complete answer:
Acid hydrometer is a device used in automobiles to measure specific gravity of sulphuric acid present in lead acid batteries at a certain temperature. From this knowledge we can get sulphuric acid concentration which in turn gives information about how much discharge the battery is.
It is based on the principle of Archimedes Principle, which states that if any object is pressed in liquid with a force then water exerts an upward buoyant force on the object which is proportional to the amount of liquid it displaced.
Acid hydrometer also known as acidometer as both have same defined work.
So the correct option is, [B] Acidometer.

Note:
It should be remembered that lactometer is used to measure density of milk, its name originated from lactobacillus bacteria which converts hot milk into curd overnight. Barkometer is used to measure atmospheric pressure, where pressure is defined as the force exerted by the atmosphere on a unit area.