Correct choice is (a) Cranium
To explain: Bones are natural impedance providers to ultrasound and so if any organ is covered or surrounded by bones, it is not possible or very difficult to take their ultrasound. For an infant, their bones are soft and do not provide so much of a resistance to the passage of the ultrasound waves. Thus, it is possible to take an ultrasound of the brain at an infant stage but not at an adult stage.