Final answer:
Option D correctly joins ideas with an -ing modifier by using a gerund phrase to modify the subject.
Step-by-step explanation:
The sentence that correctly joins ideas with an -ing modifier (gerund modifier) is option D: Peering through the microscope, my sister observed the bacteria she received last week from a veterinarian's office. In this sentence, the gerund phrase 'peering through the microscope' modifies the subject 'my sister' and indicates the action she was performing while observing the bacteria. The phrase is set off by commas to show that it adds additional information to the main clause.
Learn more about Using gerund modifiers to join ideas