"Had decided" is the correct option because it is in the past perfect tense, which is used to describe an action that was completed before another action or point in time in the past. In this sentence, the meeting was held yesterday, and the committee had already decided to make a decision by the end of the month. So, the action of deciding was completed before the meeting was held.

Option "decided" is incorrect because it is in the simple past tense, which is used to describe an action that was completed in the past, but it does not indicate when the action was completed.

Option "has decided" is incorrect because it is in the present perfect tense, which is used to describe an action that was completed at an unspecified time before now, but it does not indicate when the action was completed with respect to the meeting.

Option "was decided" is incorrect because it is in the past passive form, which is used to describe an action that was done to someone or something, but it does not indicate who or what did the action.