Select the most appropriate option to fill in the blank.

One should be careful to check the grease and oil in _______ car occasionally for its smooth functioning

The correct answer is 'one's'. 

Key Points

The sentence talks about how 'one' should be careful when it comes to checking the grease and oil in one's car in order to make sure the car functions smoothly. 

• The pronoun 'one' is already mentioned, and unless it is not possible to use the same pronoun it should be used in the sentence in order to avoid confusion. 
• The blank also implies possession, the 'car' that belongs to 'one'. 
• Thus the contraction 'one's' needs to be used - instead of 'one' - in order to imply possession. 
• 'Ones' refers to more than 1 'one', it is a plural noun, while we are looking at a singular pronoun. 

Hint

• The reader should pay specific attention to the apostrophes and the type of pronoun that is being used elsewhere in the sentence.  
• This would help them understand what type of pronoun should be used in the blank. 

Thus, the correct answer is option 4.