Problem
Every unit cell is a 1x1 square. The figure is made of exactly 11 unit cells, so there are 11 squares of size 1x1.
Just adding the cells in each row is Grade 3 addition you can do by counting the picture.
3.OA.D.9Make A Systematic ListA 2x2 square needs a full 2-by-2 block of present cells. Using rows 1-2 (top+middle): blocks at columns 1-2 and 2-3 fit, but columns 3-4 needs the missing top-right cell, so that one fails (2 squares). Using rows 2-3 (middle+bottom): both rows are full, so columns 1-2, 2-3, and 3-4 all fit (3 squares).
Composing four small squares into one bigger square is exactly what 'compose two-dimensional shapes' means in early geometry.
1.G.A.2Identify SubproblemsA 3x3 square needs three full rows of three present cells. Columns 1-3 work because all three rows have columns 1, 2, and 3. Columns 2-4 would need the top-right cell, which is missing, so it fails. That gives 1 square of size 3x3.
Drawing the 3-by-3 box on the grid shows at a glance that only the left block fits the stepped shape.
1.G.A.2Draw A DiagramTotal squares = (1x1 count) + (2x2 count) + (3x3 count). There is no room for a 4x4 square because the figure is only 3 rows tall.
Collecting the size-by-size subtotals into one sum is straightforward Grade 3 addition.
3.OA.D.9Make A Systematic ListThe total number of squares in the figure is the number of 1x1 squares plus the number of 2x2 squares plus the number of 3x3 squares.
Every square that can be traced has exactly one side length, so sorting all of them by size drops each square into one size group and leaves none out.
When a whole collection is split into groups with nothing left out and nothing shared between groups, the sizes of the groups add back to the size of the whole collection.
The three size counts combine into one definite total no matter which order or grouping you add them in, so 11 + 5 + 1 is well defined.
Changing the order in which you add the numbers does not change their sum.
Changing which two of the three numbers you add together first does not change their sum.
Count squares one size at a time, smallest to biggest, and you will never miss one - that's Grade 3 thinking you already have!