1.1 Welcome To Chemistry.

chemistry is the study of matter and how it changes.

Since chemistry is the study of matter and how it changes, chemists do exactly that.
Some of the other answers are true but are examples of what a chemists may do, not what he or she does overall.

All of the above are true.

A molecular answer to a problem is often full of useful information, since from that structure one can determine physcial and chemical answers, and with that foundation, biological or even psychological answers, for example, follow. A chemical structure is often the root answer to any matter-based topic.

Scientists define chemistry as the study of matter and how it changes to emphasive both the physical study of matter (not changing it's structure) as well as the chemical study of matter - converting it into other substances.
By changing matter we are not converting it into other elements or energy, we are simply rearranging the atoms we start with- this is chemical change. Converting one element such as hydrogen into another element such as helium (which liberates huge amounts of energy) is a nuclear process, not a chemical process.

Chemists use a very broad definition of a chemical: it is a pure form of matter,also known as a substance. It does not have to be toxic or dangerous, although in common usage it may have that connotation. It can be a single element such as gold, or a combination of elements such an methane. However those atoms must be bonded together to form a single substance: a chemical is a pure form of matter, not a mixture.

A substance is a pure form of matter.
It many be a single element or atom (in which case there are no bonds), it cannot be a mixture.