Which word completes the sentence correctly: A tuberculin skin test should be read within 24 hours _________ administration.

The choice "of" is the correct word to complete the sentence because it indicates a relationship of time or duration related to the administration of the tuberculin skin test. The phrase "within 24 hours of administration" effectively communicates that the reading of the test needs to occur subsequent to its administration, specifically within the allotted 24-hour timeframe.

Using "of" in this context clarifies that the administration event is the point in time from which the 24-hour window starts. The preposition "of" is commonly used in expressions of time to show connection and helps convey that the reading is to be done in relation to when the test was given.

The other choices do not fit grammatically or contextually. "In" would suggest being contained within a large period, which does not precisely convey the relationship intended in the sentence. "Has" is a verb and does not fit the structure required here. "On" would suggest a specific day, which doesn’t appropriately indicate the duration of time relative to the administration event needed in this situation.