In the given sentence, the phrase "our association received many letters from those adversely affected and we resolved to" indicates that there was a response planned by the association after receiving these letters.

Let's analyze the options provided:

A. Assist them: This option suggests providing help, which seems appropriate in the context of natural disaster like floods. 

B. Invite them: Inviting them doesn't make sense in this context. The association is already aware of their situation through the letters; there is no need to invite them.

C. Respond them: This phrase is not grammatically correct. The correct form would be "respond to them." However, even with the correct grammar, it's not the best option as it is vague and doesn't specify the nature of the response.

D. Pity them: This option suggests feeling sorry for them, but it doesn't imply any action or assistance. Moreover, it might be considered insensitive in this context.

E. Query them: This option means to question or inquire about something. It doesn't fit the context of the sentence where a response/action is required after receiving letters from the affected people.

The most appropriate option in this case is Assist them. This choice conveys the idea that the association planned to help or support those adversely affected by the 1980 Ogunpa floods, which is a suitable and compassionate response to a natural disaster.