The correct option is B Phloem
An aphid is a small sap-sucking insect and pierces its proboscis into the stem of the plant to extract sugary juices to feed itself. Since the stem is the most convenient part to access they are mostly found at such regions although aphids can feed from the roots, leaves etc. Plants transport sugars through the phloem cells and hence an aphid mainly pierces its proboscis into the phloem to suck up these sugar-rich juices. So, the correct answer is 'Phloem'.