Sunspots, now recognized by scientists as areas of reduced solar surface temperature that are caused by magnetic activity, were once thought to be planets due to the___________of astronomical knowledge at that time.

first lets make pair of synonyms from options they are (lack,scantness) (difficulty,obstruction)

here green portion is main clue it says that once thought to be plantes due to ________ of astronomical knowledge at that time.

once thought like that means there is no knowledge at that time

so (lack,scantness) are the answers

option C,D are correct