Choose the option which best corrects the sentence-
a) The President’s house is guarded by several units of police.
b) The Presidents house is guarded by several unit's police.
c) The President’s house is guarded by several unit's police.
d) No change.

Hint: Punctuation marks play an important role in grammar. Even a single punctuation mark can change the meaning of a whole sentence.

Complete answer:
In the given question, we have to check the punctuation marks carefully. The only punctuation mark used here is an apostrophe. The apostrophe indicates possession.
The apostrophe is used with nouns in two ways:
1. Apostrophe before 's' – to show possession with a singular noun or a plural noun that does not end in ‘s’
2. Apostrophe after 's'- to show possession with a plural noun which ends with 's'
In the given question, we have to find out which sentence has the correct usage of the apostrophe.
Now, let us examine all the given options to find out the correct answer :
Option ‘b’ and ‘c’ has incorrect usage of an apostrophe. We cannot use it after ‘unit’ in the particular sentence, else, it will be grammatically incorrect.
Option ‘a’ has the correct usage of an apostrophe, and it is grammatically and logically correct.
Option ‘d’ can be the answer only if the other three options are incorrect or not suitable for the particular question. Here, the option ‘a’ is the correct option.

Note:
The two rules have to be followed in any manner for questions that include an apostrophe. The President cannot have ‘s’ following it as there is only one President. It is a singular unit.