Answer:
The "a" Option is correct.
Step-by-step explanation:
The "COUNTIF" function counts every cell that, given a condition (value), suits into it. As you want to know the number of cells that contain a value of at least 50, the condition must be properly written to get the correct answer. Unless it is a cell value (e.g. B3), the condition must always be written with quotes (""). So, the options b and c are automatically discarded.
The d option appears to be correct, but it's not. If the condition is written ">50", the function will count every cell with a value above 50. But we're searching values at least (including) 50. So the correct answer is the a option.