Correct answer: A. Isobath
Explanation: The direct answer is A, Isobath. An isobath is a line joining places having the same depth below sea level. Ocean-floor maps use depth measurements, just as land maps use height measurements. The idea is simple: if several points are 2,000 metres deep, one line may join them; that line is an isobath. Option A is correct because “iso” means equal and “bath” relates to depth. Option B, Isohyet, joins places receiving equal rainfall, not equal depth. Option C, Isobar, joins places having equal atmospheric pressure. Option D, Isotherm, joins places having equal temperature. Thus, depth means isobath. Remember: bath sounds like bathymetry, the measurement of underwater depth.