If I ______ a bird, I would fly all over the world.
Correct: C
This is a Second Conditional sentence, used for hypothetical or imaginary situations in the present or future. The 'if' clause uses the past simple, and for the verb 'to be' in such hypothetical conditions, 'were' is used for all subjects (even 'I' or 'he'). The main clause uses 'would' + base verb.