D. The simplest solution is to treat each line independently. Ignore everything else. Find the pattern in columns and rows. Is the line there? Is it not there?
Give it a try! :)
Solution rule and steps below
Rule: In all rows and all columns, each line will appear exactly once or twice. Never zero, and never three times.
How to Find Clues: In our solution column and row, we need lines that appear twice, or lines that are missing twice. If a line appears twice, it will not appear in our solution. If a line is missing twice, it will appear in our solution. Ignore everything else.
Solve:
Step 1 - Narrow our answers
Look down column 3 for a common line or common missing line.
There are two top line segments, so we cannot have an answer with a top line. That eliminates A, B, E and F, leaving only C and D as possible solutions.
Step 2 - Find our solution
Look across row 3 for a common line or common missing line. The left vertical line is missing twice, so our answer must have a left vertical line. C has no left vertical line. Therefore the answer is D.
Step 3 - Verify our solution
We can then "work backwards" and verify the solution both horizontally and vertically for all other individual lines