Answer:
D: I have a bicycle; I'll be riding in later today.
a semicolon connects to phrases that go in succession or are related..
the first independent phrase is "I have a bicycle" and the second one is "I'll be riding it later today"
a thing i have noted is that if you can replace the semicolon ';' with the word 'anyways' and the phrase makes sense then the semicolon was correctly placed.