How to filter an array of JSON objects with jq?

Use the select filter of jq:

jq '.zk_kafka | .[] | select(.InstanceType == "t2.medium")'

Use the --arg option to pass an argument to the query to avoid injections.

jq --arg instance "t2.medium" '.zk_kafka | .[] | select(.InstanceType == $instance)'

jq has a manual, a tutorial and a cookbook.