a plane figure having six sides is a hexagon
A plane figure having four sides and four angles
In two dimensions, lines can be parallel, angles cannot. So no plane shape has parallel angles.
Yes, it can... but does not have to. A parallelogram is a four-sided plane figure with opposite sides parallel. So as long as it has opposite sides that are parallel, it is a parallelogram.
A trapezoid is a 4 sided quadrilateral having a pair of parallel lines of different lengths
square or rhombus
A square is a plane geometric figure having 4 equal sides and 4 right angles.
That's a rhombus. With an additional constraint on the angles, it's also a square.
a decagon
Triangle
trapezium
Triangle is plane with three sides and three angles. The sum of its inside angles is equal to 180 degrees.