The Electric field is given by $\overrightarrow{\mathrm{E}}=\frac{\overrightarrow{\mathrm{F}}}{{\mathrm{q}}_0}$, here the test charge 'q₀' should be
a) Infinitesimally small and positive
b) Infinitesimally small and negative

Given data: Test charge = $q_0$

Concept Used: Electric field 

Detailed Solution: Electric field: $E = F/q_o$

Where q₀ be the infinitesimally small charge 

Test charge is always infinitesimally small  

 Sign convention: Positive implies it doesn't hamper the electric field.