△ area of △ABC: 3×4÷2=6 (square centimeter)

According to the folding, the area of △ACE is equal to the area of △ADE, with AD = AC = 3cm and BD=5-3=2 (cm).

Because the heights of △ADE and △BDE are equal, AD: BD = 3: 2, therefore, the area of △ADE: the area of △BDE = 3: 2.

Because the area of ACE is equal to the area of delta delta △ADE,

So the area of △BDE is 28 of the area of △ABC, that is, 14.

So the area of the triangle BDE is 6× 14= 1.5 (square centimeter).

A: The area of the triangle BDE is 1.5 cm 2.