Answer: D

Solution:

Read each triangle's side lengths from the grid. A horizontal or vertical side is counted directly, and a slanted side spanning $a$ units across and $b$ units up has length $\sqrt{a^2 + b^2}.$

Top-left triangle: its two slanted sides each span $1$ across and $2$ up, so each has length $\sqrt{5},$ with a base of length $2.$ Two equal sides, so it is isosceles.

Top-middle triangle: it has a vertical side of length $2$ and a horizontal side of length $2$ (its third side is the slant $2\sqrt{2}$). Two equal sides, so it is isosceles.

Top-right triangle: its sides are $2,$ $\sqrt{5},$ and $\sqrt{13},$ which are all different, so it is not isosceles.

Bottom-left (wide, flat) triangle: its two slanted sides each span $3$ across and $1$ up, so each has length $\sqrt{10}.$ Two equal sides, so it is isosceles.

Bottom-right triangle: two of its sides each span $2$ and $1$ (giving length $\sqrt{5}$), while the third is $\sqrt{10}.$ Two equal sides, so it is isosceles.

Four of the five triangles are isosceles.

Thus, the correct answer is D .