Naked Single
When only one candidate is left in a cell, that digit is decided.
All candidates have been entered in the empty cells.
Because row B contains 1, 4 and 9, 1, 4 and 9 cannot go in cell A. Because column C contains 2, 3, 6 and 8, 2, 3, 6 and 8 cannot go in cell A. Because block D contains 5, 5 cannot go in cell A. Cell A is 7.