The correct answer is two hydrogen bonds between adenine and thymine (the bond is thus weaker and longer) and three between cytosine and guanine.
A hydrogen bond is a bond formed between a hydrogen (H) atom and electronegative atom or group, such as nitrogen (N), oxygen (O), or fluorine (F).
Hydrogen bond is found in the double helical structure of DNA between its base pairs. Guanine is complementary linked with cytosine and adenine with thymine. Thanks to hydrogen bonds one complementary strand is connected to the other and replication is enabled.