Answer : 1. "1"
Explanation :

acceptable will replace accepted/by will replace to because - accepted (Verb) : to agree/approve of something acceptable (Adj.) : agreed/approved by most people in a society; agreeable; capable of being accepted Look at the examples given below : The princess accepted the offer. (Active Voice) The offer was accepted by the princess. (Passive Voice) The offer was acceptable to the princess. Hence, acceptable to/accepted by will be used. Hence, Unless it is accepted by/ acceptable to both the parties, an is the right usage