Which of the following cations gives colourless (white) flame?

The correct option is A Magnesium
Flame Test is an analytical test used to determine (or a hint) the presence of cation by taking the salt in $Pt$ wire and dipping it into concentrated $HCl$ and then holding it in a hot (non-luminous) Bunsen flame, the colour of respective ions appear.

$C{a}^{2+}⟶$ orange-red

$M{g}^{2+}⟶$ no colour

$K^{+}⟶$ lilac (pink)

$B{a}^{2+}⟶$ pale green