Apsychologist plans to study the development of "intelligence" and his primary concern is whether he will actually study intelligence rather than another phenomenon, such as "reading fluency." to address this concern, he must ensure:

validity

Explanation:

The concern on setting a valid test to address the level of intelligence hs been an ongoing debate ever since IQ tests appeared:

For most people today there is a consensus that a general intelligence factor exists, and to measure it, a reliable test needs to be consistent over time and also show validity to be able to measure intelligence rather than "reading" "memory" or others side components of intelligence.

The IQ tests are today starting to take into account several formulas for also measuring the social and emotional components of intelligence.

i believe the answer is:   psychological  experiment

in this experiment, wilhelm wundt became the first person that implement scientific method in analyzing the pyschological phenomenon.

due to this result, wilhelm wundt also able to make psychology to be acknowledged as a separate scientific field from biology.