Answer:
It is a line.
Explanation:
Since, the general equation of,
A line is ax + by = c
A circle is

An ellipse is

A parabola is
or
A hyperbola is

Where, a, b and c are constants.
Here, the given equation is,
x + y = 5
Which is the type of ax + by = c
Hence, x + y = 5 is a line.