Final answer:
The correct form of the word that fills in the blank is 'stationary'.
Step-by-step explanation:
The correct form of the word that fills in the blank is stationary. The correct sentence would be: My dog has had real problems since he started chasing stationary. The other options, 'stationery' and 'cars', are not appropriate in this context. Stationery refers to writing materials, and cars is unrelated to the sentence.
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