The correct option is B Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
In large woody trunks, rays serve for radial conduction of solutes. In big woody trunks during secondary growth primary phloem is crushed and secondary phloem, which forms a thin layer in bark only remains active. Next year a new layer of functional secondary phloem is produced and so on every year. The radial conduction refers to the radial flow of photosynthates from secondary phloem of bark to inner regions of thick woody stems. In plants sieve tube transport organic solutes, i.e., photosynthates from source to sink.