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An isosceles triangle has one line of symmetry
a pentagon
You cannot have one parallel line. There must be another line that it is parallel to. Also, 1 symmetry is meaningless - you can have one axis of symmetry or rotational symmetry of order one. The answer will depend on which one it is.
a symmetrical shape
There are infinitely many lines of symmetry. Every line can be a line of symmetry for a suitable shape.