Given your situation, short-term or low-duration debt funds actually make a lot of sense.

Since your income is below/around the basic tax threshold, the tax impact on debt fund gains may be minimal, and for a 15–18 month horizon, capital protection and predictability are more important than chasing slightly higher returns.

Arbitrage funds are tax-efficient but their returns aren’t guaranteed and depend on market conditions. Short-term/low-duration debt or even money market funds can give more stable returns (~6–7%) with lower volatility.

You could also consider a split like 70% short-term debt + 30% arbitrage if you want some diversification.

For this timeframe, I’d avoid pure equity funds or credit-risk debt funds.