Gerund function

Hi, I'd like to know how responding is working here: what it modifies.

In January, when deep crisis and tragedy struck Haiti, TWB was ready to help. Working with a surge in volunteers, the organization was able to provide support for humanitarians responding to the crisis.

Could it be the organization was able to provide support for humanitarians who were responding/responded to the crisis or maybe responding make reference to how the organization provide support for humanitarians.

Thanks in advance,

Your first interpretation is correct: the organization was able to provide support for humanitarians who were responding/responded to the crisis.

The organization is supporting. The humanitarians are responding.

"who were responding to the crisis"

Sometimes in English, we drop words that we consider to be "understood" in favor of concise writing or speech.

The sentence you cite is exactly the same as:

"... the organization was able to provide support for those humanitarians who were responding to the crisis.