Which statement about quarks is not correct?
a. only two quarks are needed to construct a hadron.
b. an isolated quark has been observed by physicists.
c. every quark has an antiquark of opposite charge.
d. there are six quarks that fit together in pairs.

B

Explanation:

- A quark is an elementary particle that combine to form any matter. The combination of two quarks e.g proton and neutron combine to form atomic nuclei. This is the most smallest type of stable hadron in existence. Other hadrons contain 2 or more quarks for stability.

- The electrical neutrality of a quark is only maintained by the associating anti-quark. Kind of like a dipole. Without the existence of anti-quark a quark is highly unstable and will find an anti quark in the surrounding to mate and neutralize. Hence, isolation of an anti-quark is not possible.

- Their are 6 types ( flavors ) of quark. 2 up and 1 down constitutes 3 quarks. While the other three are their anti-quark pair.