Acause of the first crusade was the
a. conquest of constantinople by the ottoman turks
b. invasion of spain by the muslim arabs
c. attack by the muslims on rome
d. conquest of asia minor by the seljuk turks

The correct answer would be D - conquest of Asia Minor by the Seljuk Turks.

At the time of the First Crusade and before it, the Seljuk turks and other smallers fractions were causing havoc and distress among different populations. Eventually they also started causing distress and killing innocent christians (and conquering different lands in Asia Minor) in larger numbers which eventually led (among other things) to the First Crusade.