public class Persist
{
    public string[] data;
}

<form method="POST">
    <select class="form-control w-100" id="data2" name="data[2]" onchange="openOptions()">
        <option value="N"> Não analisar</option>
        <option value="S"> Sim</option>
    </select>

    <select class="form-control w-100" id="data5" name="data[5]" onchange="openOptions()">
        <option value="N"> Não analisar</option>
        <option value="S"> Sim</option>
    </select>

    <select class="form-control w-100" id="data8" name="data[8]" onchange="openOptions()">
        <option value="N"> Não analisar</option>
        <option value="S"> Sim</option>
    </select>
</form>

private static string Save(Models.Persist p)
{
    var teste = p.data[0]; // Dá erro
}