The correct option is A who attended
In the underlined part "whom attended", "whom" is a relative pronoun which functions as the object of a verb or preposition. In simple words, it receives an action and hence, will follow the verb or preposition. In the underlined part, a pronoun is required which performs the action "attended".
Hence option D is incorrect.
Option A: 'Who' is a pronoun that can perform as a subject. Hence A is the correct option.
Option B: 'Whom have attended' is also wrong as 'whom' does not perform an action.
Option C: Adding the preposition 'with' does not improve the sentence as a subject pronoun is required to perform the action.
Hence B and C are incorrect