Blessed is he who has found his work.
This statement is a sentence fragment and lacks a main verb. However, we can assume that the main verb is implied and understand the meaning of the sentence. The correct option to complete the sentence is "B" - who.

Explanation:
- The sentence is expressing a blessing or praise for someone who has found their work.
- The pronoun "he" refers to a specific person, and the word "who" is used to introduce a relative clause that provides additional information about that person.
- In this case, the relative clause is describing the person who has found their work, indicating that they are the one who is blessed.
- The use of the word "who" is appropriate here because it refers to a person, rather than a thing (which would require the use of "that" or "which").
- The sentence could be rephrased as "He is blessed, the one who has found his work."
- By using "who" instead of "that" or "which," the sentence emphasizes the personal aspect of the individual who has found their work and suggests that this is a significant achievement for them.
- The sentence fragment structure may be seen in literary or poetic contexts where brevity and a certain rhythm or impact is desired.