When resistors are connected in series, their resistances add up. In this case, if three equal resistances are equivalent to 90 ohms, it means that each individual resistor has a resistance of 30 ohms (since 30 + 30 + 30 = 90).

When resistors are connected in parallel, their equivalent resistance can be calculated using the formula 1/Req = 1/R1 + 1/R2 + 1/R3 + ..., where Req is the equivalent resistance and R1, R2, R3, ... are the resistances of the individual resistors.

In this scenario, since all three resistors have the same resistance of 30 ohm, we can rewrite the formula as 1/Req = 1/30 + 1/30 + 1/30. Simplifying this equation gives us 1/Req = 3/30.

To find the equivalent resistance, we take the reciprocal of both sides of the equation: Req = 30/3. Simplifying further, we get Req = 10 ohms.

Therefore, the correct answer is option 1, 10 ohm.