A. It depends on your investment time horizon. Also, it depends on how large is the amount.
If this is for long term investment, and the amount is not huge, you should invest it all at once as a lump sum. Because over the long term, the market will be up. If you are concerned the market might be down in the near future, you should diversify your investment into different asset classes so they don't all go up or down at the same time, and you will rebalance periodically to take advantage of the downs of some of the asset classes.
When dollar cost averaging is applicable? It's usually for a specific amount at specific time intervals, such as a portion of your paycheck needs be invested regularly. That works best for small amounts as you accumulate them, not for a large lump sum available now, because if you do DCA for the lump sum, you will face the question - over how long a time period? is half year long enough? 1 year period? 5 year period? Remember money not invested would be earning nearly nothing.