Data is technically the plural of datum, so, the “correct”
pronunciation is dayta. This is also the most widely used
pronunciation.
It is also pronounced Dah-tah
in Australia and Boston area.
In modern
non-scientific use it is generally not treated as a plural.
Instead, it is treated as a mass noun, similar to a word like
information, which takes a singular verb. So as long as you are
not using it in a scientific context, it is usually fine to use
“this data is”.
Strictly-speaking, data
is a plural term. For example, if we are following the rules of
grammar, we should not write "the data is" or "the data shows"
but instead "the data are" or "the data show".
As
usage has changed over time, Oxford Dictionary indicates
both pronunciations are now acceptable.