Elements having valency 'one' are :
(a) always metals
(b) always non-metals
(c) always metalloids
(d) either metals or non-metals

Correct Answer: (d) either metals or non-metals

Explanation: Elements with valency 1 can be either metals or non-metals. When an atom loses one electron to gain stability, it is a metal. When an atom gains an electron to complete its octet, it is a non-metal.