Choose the best option which best corrects the underlined part of the sentence-
The bridge was sturdy, flexible and strong: it withstood the storm well.
a) Sturdy, flexible, strong: it
b) Sturdy, flexible and strong – it
c) Sturdy, flexible, strong; it
d) No change

Hint: The given sentence is a compound sentence. It is a sentence containing two or more coordinate independent clauses, usually joined by one more conjunctions, but no dependent clause.

Complete answer:
 If two clauses are connected with a coordinating conjunction, it's a compound sentence. If two clauses are connected with a subordinating conjunction, it is a complex sentence. Here, the mistake in the given sentence is of punctuation marks. Check which one of the four options has the correct usage of punctuation marks.
Option 'a' - This is the wrong option, it is grammatically incorrect.
Option 'b' - This is also grammatically incorrect. So, we will eliminate this option straight away.
Option 'c' - A semicolon is used to show bigger separation when there is already a comma within a sentence for smaller separations. 'Sturdy', 'flexible', and 'strong' are more than one predicate in the sentence, thus, a semicolon is used after them to show bigger separation.
Option 'd' - This would be chosen if the other three options are incorrect or the given statement is already correct. The given statement is already correct.
Hence, option 'd' is the correct option.

Note:
Sometimes no change is needed in the given sentence, the underlined part of the compound sentence is already correct, it doesn't need any changes.