Explanation
Using opportunity team and creating an assistant field, using Apex to share opportunities with the assistant agent is the optimum solution to meet the requirements. Opportunity teams are groups of users who work together on sales opportunities. By creating an assistant field on the opportunity object, the sales reps can specify an assistant agent who can help them with their deals. By using Apex, the system can automatically share the opportunity with the assistant agent based on the value of the assistant field, and remove access from the previous assistant if the field value changes. Option A is incorrect, since using share group to share opportunities with the assistant agent would require manual configuration and maintenance. Option C is incorrect, since using sharing rule to share opportunities with the assistant agent would not allow dynamic sharing based on the assistant field value. Option D is incorrect, since using Apex sharing to share and unshare opportunities with the assistant agent would be similar to option B, but without using opportunity teams.