When you try to remember a series of digits, you are using:

a. flashbulb memory.
b. echoic memory.
c. long-term memory.
d. short-term memory.

The answer is D. Short-term memory.

Explanation:

Short-term memory.  active memory, or primary memory is where information gathered and may be recalled briefly, if not rehearsed. Usually, the brain gathers this information through hearing, even translating visual signals to auditory information. It is also very limited as it's known to only hold a maximum of seven + two kinds of data in a single instance, and lasts 20-30 seconds only. This brief memory may be verified by letting the person recall the series of digits backwards and only recalling the last 3 digits.