Water boils at 100°C at sea level. If the water in this experiment did not boil at 100°C, what could be the reason?

You are not at sea level.

Explanation:

Assuming empirical observation shows boiling point 'below' 100°C then one concludes boiling experiment is being conducted above sea level where air pressure is lower than sea level air pressure. Generally, it is accepted in most literature publications that atmospheric pressure decreases ~1-inch per 1000-ft increase in elevation.

By definition, the boiling point of a liquid occurs when the atmospheric pressure equals fluid vapor pressure. Thus, if experimentally a water sample  boils less than 100°C then one can only conclude the atmospheric pressure is lower than that needed to boil water at sea level atmospheric pressure; i.e., observed boiling point is at elevation greater than sea level.

One can calculate the boiling point of a liquid relative to sea level boiling point by using the Clausis-Clapeyron Equation.

ln(VP₂/VP₁) = ΔH(vap)/R(T₁ - T₂/T₁·T₂) solve for T₂ given Heat of Vaporization of water (ΔH(vap) = 28.2 Kj/mol) at specified temperature (T₁ at sea level = 273 Kelvin - standard) and R = 0.008314 Kj/mol-K).

the answer is c i believe