Answer:
Circle
Explanation:
The strip is formed according to the rule:

The length of each box with each figure is 1 inch.
The length of the string is 74 inches, so there will fit 74 boxes with figures. So, you need to know which figure will be last.
1st, 4th, 7th, ... 3n+1 th box contains triangle,
2nd, 5th, 8th, ... 3n+2 th box contains circle,
3rd, 6th, 9th, ... 3n th box contains square.
Now,

this means 74th box contains circle, because we add 2 to the product of 3 and 24.