To find the appropriate modes of communication that can be employed in a classroom for effective teaching, let's analyze each option given:

1. Option (a): Top-down - This mode of communication usually involves breaking down a system to gain insight into its compositional sub-systems in a reverse engineering fashion. It is not typically a mode of communication used in classrooms.
2. Option (b): Iconic - Iconic communication involves using images or icons to convey ideas. This method is effective in classrooms as it helps in visually representing information and making complex ideas easier to understand. Therefore, it is an appropriate choice for classroom communication.
3. Option (c): Associational - Associational communication allows for the linking of new information to prior knowledge, thereby enhancing understanding and retention. It is an effective teaching strategy in classrooms.
4. Option (d): Dissociational - Dissociational communication involves separating ideas or concepts, which is not a commonly used method for effective teaching in classrooms.
5. Option (e): Symbolic - Symbolic communication uses symbols, signs, or letters to convey ideas. It is widely used in classrooms, as symbols form the basis of written language and mathematics, making this mode highly relevant for teaching.
6. Option (f): Abstract - Abstract communication involves conveying complex ideas that are not concrete. Though valuable, it is not frequently used on its own for classroom teaching without other supportive modes.

From the above analysis, the most effective modes of communication for classroom teaching are iconic, associational, and symbolic. Thus, the correct and most appropriate option is (b), (c), and (e). These modes help to visually represent content, relate new knowledge to existing knowledge, and use symbols for concrete understanding, respectively.