Step 1: Understanding the Concept:
"Foster" means to care for, raise, or promote the development of someone or something.
Step 2: Detailed Explanation:
"Encourage" means to give support, but "nurture" more precisely captures the care and development aspect of "foster." "Fabricate" means to invent falsely. "Forment" (actually "foment") means to instigate trouble. "Nurture" is the best match.
Step 3: Final Answer:
Thus, "Nurture" is the correct synonym.