It depends whether you want to test their pure R ability, or their statistical ability. But overall I feel that the above questions test knowledge rather than understanding. Knowledge can be easily ascertained by quickly Googling. Understanding is what shows proficiency at something.
For their R ability questions as the above are okay, though I think I'd ask questions such as:
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Create a vector of a certain length, with samples from a Normal distribution?
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Find the arithmetic mean, the median and the IQR from a sample of data?
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Create a function that returns the minimum from each row of a matrix?
For wider statistics and programming ability I'd present them with a dataset and ask them to perform exploratory data analysis, create visualisations and get some conclusions from data. This allows you to see how they approach tasks, their understanding of what they are doing and the quality of coding.
Though, I appreciate, depending on time and infrastructure, all of this might not be possible.
I would also say that questioning/marking should be done by people with R knowledge, if you are trying to determine an applicant's level of R knowledge.