Answer:
d) pertinent
Explanation:
Assuming that this is the same 'line 7' from your other question that read '“Although air is mostly nitrogen, this gaseous form of nitrogen is inaccessible to plants and must be “fixed” from the air into ammonium to render it biologically relevant”) ("Although . . . relevant")' , i believe the answer is d) pertinent [in other words, suitable or fitting]
