SELECT "Prod. code" FROM table_3515 WHERE "Episode #" = '78'
What is the production code for episode #78?
CREATE TABLE table_3515 (
    "Episode #" text,
    "Season Episode #" text,
    "Title" text,
    "Directed by" text,
    "Written by" text,
    "Original air date" text,
    "Prod. code" text
)