Suppose, I buy XRP and set limit of 15% fluctuation. The coin should immediately be converted into Monero if this happens. Is there any exchange which offers this function ?
Fluctuations you mean volatility? Volatility means up or down, do you mean price up or price down? I don't think this can be automated but trading bot can do the magic through API connection, I don't think inbuilt trading bot like Kucoin will work but an external bot will do the magic.
Alternatively, you can calculate it yourself using trade set up. Exchanges like Binance has trading future tools, you can simply use their tools to set up your sell order of 15%, when you trace the price and have that 15%, you can just use that value as sell order and if that price is reached, it will automatically sell your coins for you with out any bot or even monitoring the trade.
However, will you have xmr at such price fluctuations, is there something I'm missing? There has not been any reason cent development around xmr, perhaps you should look into other altcoins that are having some speculations and updates, 15% is a little thing to get from some of these altcoins like Solana, Avalanche and xpr, they are having some recent development and new updates.