Greg's current hypothesis has several issues that can be identified. Here are the applicable points from your options:
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It isn't written in the form of a statement. - A hypothesis should be presented as a statement rather than a question. For example, it could be rephrased to something like, "Dandelion flowers will bloom best with X hours of sunlight per day."
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It is too broad, and not testable. - While the concept of sunlight and its effects on flower growth is valid, the hypothesis as phrased does not specify the parameters necessary for testing (e.g., what "best" means, how it will be measured).
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It doesn't define a dependent variable. - The hypothesis does not specify what the dependent variable is (e.g., growth rate, number of blooms, health of the flowers), making it difficult to measure outcomes.
The other options regarding testing a wider variety of flowers or providing enough evidence are not necessarily relevant to the current formulation of Greg's hypothesis.