The city performed its annual testing of lead levels in the drinking

Conclusion: it’s perfectly safe for the city’s residents to drink all the tap water they want.
Premise : the lead levels, even in the ones that exceeded federal standards, were still too low to pose any immediate health threat

There is a gap btwn the conclusion and premise. Immediate threat does not mean the water is safe.

(A) The city’s residents often disregard federal guidelines when it comes to public health issues because the standards are overly stringent.--Irrelevant or not necessary

(B) The most dangerous contaminant in drinking water is lead.-- Irrelevant

(C) Statistical sampling is not the most accurate basis for determining health threats even though the federal government and the city must depend on them. -- Incorrect, this is not necessary to conclude

(D) Lead levels even slightly above federal standards do not have long-term effects on people’s health. -- Correct

(E) People feel safer when they heed warnings from the federal government even if they are not entirely accurate. --Irrelevant

OFFICIAL EXPLANATION

Answer: D

STEP 1: Read the question and identify your task.
This is an Assumption question. For this first question you must find the assumption that the argument depends on in order to hold true.

STEP 2: Read the argument with your task in mind.
The argument is about annual tests of the lead levels in drinking water. The conclusion is that the water is safe to drink, even though some samples
exceeded federal standards, because the levels were still too low to pose any risks.

STEP 3: Know what you’re looking for.
The correct answer will address the relationship between the federal standards and the amount of lead that causes health problems.

STEP 4: Read every word of every answer choice.
Answer A says the standards are too stringent, but the lead levels still may be harmful to people’s health, and whether they disregard the guidelines or not, they take a risk by drinking the water. Answer B seems to weaken rather than support the argument, and that is not what the question is asking. Answer C calls into question the method of testing and leaves open the option that the situation might be worse than stated, possibly leading to the opposite recommendation, that people should not drink the water. For answer D, if lead levels slightly above federal standards are still not harmful to people’s health, then it is valid to say that drinking the water is still safe for the city’s residents. This very much supports the argument and is critical for the argument’s conclusion. Answer E has nothing to do with the lead levels or drinking the water. Rather, it gives an irrelevant observation about human behavior. Therefore, answer D is the correct choice.