In the following question, the sentence is given with a blank to be filled in with an appropriate phrase. Select the correct alternative out of the four and indicate it by selecting the appropriate option. That is _____ cars I know.

Introduction / Context:
This fill in the blank question tests your understanding of comparative and superlative structures in English, especially the pattern used when we compare one item with a larger group. The sentence talks about a particular car and how bad it is compared to other cars that the speaker knows. The phrase must fit grammatically and logically with the plural noun cars that follows.

Given Data / Assumptions:

• Sentence: That is _____ cars I know.
• Noun immediately after the blank: cars, which is plural.
• Options: the worst, worst, mostly worst, one of the worst.
• The likely intended meaning: the car is among the worst cars that the speaker knows.

Concept / Approach:
When we compare a single item with a group of many and want to say that it belongs to the bottom or top part of that group, we use the structure one of the plus superlative plus plural noun. For example, one of the best films, one of the tallest buildings, or one of the worst mistakes. In this sentence, because cars is plural and we are referring to a particular car as part of that group, the natural structure is one of the worst cars. The other options do not fit both the grammar and the intended meaning correctly.

Step-by-Step Solution:
Step 1: Note that the noun after the blank is cars, a plural noun.Step 2: Decide whether we are talking about one car or all cars in general. The word that is points to one specific car.Step 3: Recall the pattern for comparing one item against a group: one of the plus superlative plus plural noun.Step 4: Apply this pattern: one of the worst cars fits perfectly with the structure That is one of the worst cars I know.Step 5: Check each option inside the sentence to ensure grammatical correctness and natural sound.Step 6: Confirm that only one of the worst produces a complete and idiomatic sentence.

Verification / Alternative check:
Insert the chosen phrase fully: That is one of the worst cars I know. This sentence means that among all the cars the speaker knows, this particular car belongs to the group of very bad ones. It is a common and natural way to express a strong negative comparison. If we try That is the worst cars I know, we see an agreement problem between the worst and cars, since the worst used in this way normally goes with a singular noun, such as the worst car. Similarly, That is worst cars I know is clearly ungrammatical because it lacks the or one of the before the superlative.

Why Other Options Are Wrong:
The worst: This would be correct if the noun were singular, as in That is the worst car I know. With the plural cars, however, the phrase the worst cars refers to more than one car, which conflicts with That is, pointing to a single car. Worst: Using worst alone without an article or supporting phrase is incorrect here and sounds incomplete: That is worst cars I know does not follow standard English patterns. Mostly worst: This phrase is not a standard English expression. Mostly is used with adjectives in other ways, but mostly worst cars sounds ungrammatical and awkward.

Common Pitfalls:
Many learners mix up the structures for absolute superlatives and part of a group comparisons. They may know that the worst is the strongest form and try to use it everywhere, even when the sentence actually requires one of the worst. Another common mistake is ignoring subject noun agreement and just checking whether the phrase looks strong. To avoid such errors, always check the number of the noun following the blank and ask whether you are picking out the single best or worst item in the whole group, or simply saying that the item belongs to that extreme group. If it is part of a group, one of the plus superlative plus plural noun is usually required.

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