Answer:
The sentence that uses the semicolon properly is:
I never know what to get him for his birthday; perhaps I will get him a bicycle.
Step-by-step explanation:
The reasons behind this answer are two: First, semicolons are used to unite two independent clauses or in other words. Two sentences can exist without further information about the topic. Second, there is an adverb that separates them in our case "perhaps" is that adverb. Thus, the chosen option is the correct answer.