Recall characteristic flame test colours

In flame tests, different metal cations impart characteristic colours:

• $Cu^{2+}$ : green flame, often with a blue centre
• $Sr^{2+}$ : crimson red
• $Ba^{2+}$ : apple green
• $Ca^{2+}$ : brick red

Match the observed flame colour

The observation given is:

$\text{green flame with blue centre}$

This is the characteristic flame of copper salts.

Check each option

• A: $Cu^{2+}$ — matches the observed flame colour. ✅
• B: $Sr^{2+}$ — gives crimson red flame, not correct. ❌
• C: $Ba^{2+}$ — gives apple green, but not typically described as green with blue centre. ❌
• D: $Ca^{2+}$ — gives brick red flame, not correct. ❌

Final answer

Therefore, the cation present may be:

$\boxed{Cu^{2+}}$