The answer is – both.
The word “data” is a Latin word. It is the plural of “datum”.
“Data” means facts or information; “datum” means one fact or a single item of information.
“Data” and “datum” are usually used to refer to statistical information or information subject to analysis.
“Data” is used far more commonly than “datum” and in a wider range of contexts.
“Datum” is unlikely to appear outside of specialist scientific or academic writing.
As “data” is a plural countable noun in Latin, many people take the view that it should be used in the same way in English. Thus it requires plural verb forms, pronouns and quantifiers, e.g.:
Many of those data have already been entered into the system.
When we have received the data we can start to analyse them.
There are very few data in the set.
This usage is practical for scientific or academic writing because it allows for the use of the singular “datum”.
However, it is increasingly common to use “data” as a singular uncountable noun, as follows:
Much of that data has already been entered into the system.
When we have received the data we can start to analyse it.
There is very little data in the set.
This usage doesn’t really allow for the use of the singular “datum”, so may lack precision in certain contexts.
Usage of “data” as a singular uncountable noun – in the same way as “information” – is now generally accepted in everyday English, so much so that using the word as a plural countable noun can sound incorrect. However, in much scientific and academic writing, where precision is obviously more important, it still tends to be used as a plural countable noun.
It is your choice how to use it in business or legal writing. My preference would be – as always – to use the everyday English version – “data is” – and that is increasingly the preference of contemporary grammarians. But your choice may depend on the context: if you’re writing a quick email to a native English speaker – use “data is”, or if you’re drafting a formal legal opinion on the results of a specific data analysis – use “data are”.
Also see my post on How to use the word “information”