To solve this, we first find Saurabh's position from the left end.
Since Saurabh is 18th from the right end and there are 40 students in total, his position from the left end is:
\[
40 - 18 + 1 = 23.
\]
Sheela is 5th to the right of Saurabh, so Sheela's position from the left is:
\[
23 + 5 - 1 = 26.
\]
Thus, Sheela's rank from the left end is 26.
Therefore, the correct answer is "26."