Fill in the blank: "That is ______ cars I know." Which option correctly completes the sentence with a natural English expression?

Introduction / Context:
This question tests correct use of superlative structures with plural nouns. The sentence refers to "cars" in the plural and comments on how bad one particular car is in comparison with others. The phrase you choose must agree in number and follow natural English patterns used to express very negative opinions.

Given Data / Assumptions:
- Sentence: "That is ______ cars I know." - Options: the worst, worst, mostly worst, one of the worst. - The noun "cars" is clearly plural, so the blank must work smoothly with a plural noun.

Concept / Approach:
When we compare one item with a group and say it belongs to the very negative end of the scale, we commonly use the structure "one of the worst" plus plural noun: "one of the worst cars". If we say "the worst car", the noun must be singular. Therefore, the blank needs the phrase "one of the worst" to match the plural "cars". Options like "worst" alone or "mostly worst" are not idiomatic in this position.

Step-by-Step Solution:
Step 1: Notice that "cars" is plural and that the demonstrative "That is" refers to a single car within a group. Step 2: Recall the pattern for such comparisons: "That is one of the best books" or "She is one of the tallest players". Step 3: Apply this to negative comparison: "That is one of the worst cars I know." Step 4: Test option A: "That is the worst cars I know" is wrong because a singular verb "is" would require "car", not "cars". Step 5: Test option B: "That is worst cars I know" is grammatically incomplete without an article or determiner. Step 6: Test option C: "mostly worst" is not a standard English phrase and sounds unnatural. Step 7: Conclude that option D "one of the worst" fits both grammar and idiomatic usage.

Verification / Alternative check:
Compare with similar sentences: "That is one of the fastest trains I know" or "This is one of the hardest exams students face." These are perfectly natural patterns. Replacing "fastest" or "hardest" with "worst" and "trains" with "cars" gives exactly the structure used in the correct answer. The plural noun after "one of the" is essential and fully matches the form in this question.

Why Other Options Are Wrong:
"The worst" in option A would require a singular noun ("the worst car") or a different verb ("Those are the worst cars"). "Worst" alone in option B lacks a determiner and does not fit the standard patterns. "Mostly worst" in option C is not used in standard English and mixes adverb and superlative incorrectly.

Common Pitfalls:
Learners often forget to adjust the noun from singular to plural after "one of the". A helpful rule is that "one of the" is always followed by a plural noun, because you are selecting a single member from a group. Remember also that superlative adjectives in such structures retain "the" as part of the phrase "the worst", "the best", or "the tallest".

Final Answer:
The correct phrase is "one of the worst", giving "That is one of the worst cars I know."