(a) Bond line structure:
IUPAC name: 2,2,3-Trimethyl butane
Condensed formula: `(CH_(3))_(3)C CH(CH_(3))_(2)` or `Me_(3)C CHMe_(2)`
(b) Bond line structure:

or
IUPAC name: 2,2,3,3-Tetramethyl butane
Condensed formula: `(CH_(3))_(3)C.C(CH_(3))_(3)` or `Me_(3)C.CMe_(3)`
(c) Bond line structure:

or
IUPAC name: 2,2,3,4,4-Pentamrthyl pentane
Condensed formula: `(CH_(3))_(3)C CH(CH_(3))C(CH_(3))_(3)` or `Me_(3)C.CH(Me)CMe_(3)`
(d) Bond line structure:
IUPAC name: 2,2,3,3,4,4-Hexamethyl pentane
Condensed formula: `(CH_(3))_(3)C.C(CH_(3))_(2)C(CH_(3))_(3)` or `Me_(3)C.C(Me)_(2).CMe_(3)`