Three lines are drawn from a single comer of an FCC unit cell to meet the other comer such that they are found to pass through exactly only 1 octahedral voids, no voids of any type and exactly 2 tetrahedral voids with 1 octahedral void. Identify the line in the same order.

Atoms are found in both the face centre and the corners of a face centered cubic (FCC). Tetrahedral voids are found on body diagonals, body centre, and octahedral voids are found on edge centres. On the body diagonal, two tetrahedral voids are located (1/4)^th of the way from each corner. 

Therefore, it must be the edge if a line connecting two opposite corners only goes through one octahedral void.

A line must be diagonal if it joins two opposite corners without passing through any voids of any kind.

A line joining the opposite corner must be a body diagonal if it only travels through 2 tetrahedral and precisely 1 octahedral void.