Diamond. Reason: If the crease EF is set to BC to E and AD to F, AE and CF are connected, because of folding, AE = CE and AF = CF.

∠AFE=∠CFE, and ∠ BC, ∠AFE=∠CEF.

So ∠CFE=∠CEF, so CE=CF, so AE = ce = af = cf

So the quadrilateral AECF is a diamond.