In order to understand the effects of air temperature on the rate of photosynthesis, it is important to observe the soil, the amount of water and other variables. But a Celsius thermometer it is not the appropriate tool to measure pH (differently than what is said on number 4). Also, an electronic balance it's a tool that measures mass, not volume. So, both numbers 1 and 3 are incorrect, because number 1 states that an electronic balance would measure volume (but it only measures masses) and number 3 states that a metric ruler would determine the mass (but the correct tool to measure masses is an electronic balance). Number 2 states that a graduated cylinder would measure 30mL of water, and graduated cylinders realy is used to measure the volume of a liquid. Therefore, the correct answer is number 2, a graduated cylinder to measure 30 mL of water for each plant daily.