A. A total stock market index fund such as VTI will give you exposure to ALL of the stocks in the US stock market, with an extremely low fee (0.05% of asset).
However, while it gives you good diversification, it also limits your chance of beating the market level performance in the long term. To enhance your chance, you can add a small portion of your overall portfolio to small cap and/or value stocks, because they tend to perform better than the overall market over the long term.