Scientific questions must be
a. based on scientific theories
b. general
c. biased
d. testable

d.Testable

Explanation:

Science is based on the scientific method, which starts with a question. This question raises a hypothesis, and after it, a methodology to taste it. If a question can't be tested by a controlled procedure, the scientific method can't be applied.

Option a. is wrong because all "scientific theories" were generated by the previous testing of multiple scientific questions.  Options b. and c. are wrong too, because, scientific questions try to understand a phenomenon no matter its nature (if it is general, specific..).

The right option is d.testable, scientific questions must be testable, reproducible and refutable.

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