Answer
4
Explanation
The significant figures are counted from the left hand side to the right hand side.
The zeros on the left hand side are not considered as significant. The zeros on the right hand side maybe or may not be significant. It depends on what the question requires.
The zeros between the none zeros are always significant.
3.55 is in 4 significant figures.
In to one significant figure it will be 4.00 or simply 4.
