18) D
We need to calculate the maximum number of cards needed. So we need to use most of the space available on each rectangular card.
2 ends of the H (2 cm each) can fit in the centre of another H (4 cm each).
Please draw them out as it will take longer to visualise.
Soon, you will realise that we can have 6 rows of 3, 2, 3, 2, 3, 2.
Drawing them out:
1 1 1
1 1
1 1 1
1 1
1 1 1
1 1
This gives us = 3 + 2 + 3 + 2 + 3 + 2 = 5 + 5 + 5 = 15 H shaped cards.