Which of the following investigation is BEST used for placental localization?

The most accurate method for localisation of placenta is ultrasonogram. Its advantages are: It carries no risk to mother and fetus. It is the only method by which edge of the placenta can be seen, which helps in deciding the mode of delivery. Placental tissue shows up an area of fine speckling. Its fetal surface appear as a fine line and is seen readily when the placenta is attached to the anterior uterine wall. When on the posterior wall it is not so easily seen unless there is a sufficient interface between the pool of liquor and the placenta. Ref: Manual Of Obstetrics (2Nd Edition) By Daftary page 233.