The most appropriate method for collecting urine for culture in case of vesicovaginal fistula is:
"Urine culture is mandatory before surgery and infection should be treated. The urine is collected by Catheterization."
In VVF
"Preoperative collection is best to be done through ureteric catheterization."  So friends undoubtedly ureteric catheterization. (Don't get confused - it is not Foley's catheterization) is the best method for collecting urine for culture in a case of VVF. This option is not given, so, we will have to look for next best option.

"Urine collected through a sterile vaginal speculum will not serve the purpose because of contamination. 
Midstream clean catch sample is also contaminated in vesicovaginal fistulas. (Option "b")
Supra pubic aspiration done after proper cleansing and draping the patient with full bladder, is easy and next best method of urine collection after ureteric catheterization. But the only prerequisite for this method of collection is 'A full bladder' which cannot be fulfilled in a case of VVF as urine continuously dribbles from the vagina and therefore, bladder is never full. (Ruling out Option "a")
By exclusion our answer is Foley's catheterization, although chances of contamination are present in Foley's catheterization but they can be reduced if proper vaginal douching is done prior to collection of urine.
Suprapubic needle aspiration
Midstream clean catch
Foley's catheterisation
Sterile speculum