Idea of flame test

In a flame test, some metal ions impart a characteristic colour to the flame because their electrons get excited and then emit light of specific wavelengths.

Check each given ion

We examine the ions one by one:

1. $\mathrm{Sr}^{2+}$: gives crimson red flame $\Rightarrow$ characteristic
2. $\mathrm{Ba}^{2+}$: gives apple green flame $\Rightarrow$ characteristic
3. $\mathrm{Ca}^{2+}$: gives brick red flame $\Rightarrow$ characteristic
4. $\mathrm{Cu}^{2+}$: gives bluish green flame $\Rightarrow$ characteristic
5. $\mathrm{Zn}^{2+}$: does not give a characteristic flame colour $\Rightarrow$ not counted
6. $\mathrm{Co}^{2+}$: does not give a characteristic flame colour $\Rightarrow$ not counted
7. $\mathrm{Fe}^{2+}$: does not give a characteristic flame colour in ordinary flame test $\Rightarrow$ not counted

Count the ions showing characteristic flame test

The ions are: $\mathrm{Sr}^{2+},\ \mathrm{Ba}^{2+},\ \mathrm{Ca}^{2+},\ \mathrm{Cu}^{2+}$

Total number: $4$

Comparison with stored answer

Derived answer = $4$

Stored correct answer = $4$

Hence, they agree.