Refer to figure.
When choosing a VFR level to fly, the semicircular rule should apply to enhance separation between VFR flights and reduce the risk of collisions. In simple words, aircraft following eastern magnetic tracks should be separated by 1000 ft with aircraft following western magnetic tracks. According to the semicircular rules:

• When an aircraft is following an eastern magnetic track between 0° and 179°, it should choose an altitude or flight level that will be odd plus 500 ft (3500 ft or FL35, 5500 ft or FL55, 7500 or FL75).
• When an aircraft is following an western magnetic track between 180° and 359°, it should choose an altitude or flight level that will be even plus 500 ft (4500 ft or FL45, 6500 ft or FL65, 8500 or FL85).

⇒ When our Magnetic Track is 090°, the pilot should choose an altitude or flight level of odd plus 500 ft (FL75 or 7500 ft).

QNH → altitude above mean sea level
QNE (1013.25 hPa) → flight level above the 1013.25 hPa datum

Let's examine the given options:

FL075 is always equivalent to 7500 ft AGL. → INCORRECT. AGL is the altitude above ground level. This cannot be determined by the altimeter and is not the same as the flight level.

FL075 indicates the altitude of an aircraft in flight. → INCORRECT. Altitude is referenced to QNH, while flight level is referenced to 1013.2 hPa. Therefore, FL075 is a flight level and not an altitude indication.

A VFR pilot, flying on a magnetic track of 090° may elect to fly at FL075 to conform to the semi-circular rule. → CORRECT. The magnetic track of 090° is between 0° and 179°. Therefore, a flight level of odd plus 500 ft can be chosen such as FL075.

An altitude of 7500 ft is indicated on an altimeter in flight when using the standard altimeter setting of 1013.25 hPa. → INCORRECT. Altitude is referenced to QNH, while flight level is referenced to 1013.2 hPa.